Why did St. Augustine and St. Thomas endorse ensoulment at 40 or 80 days?

I believe that Augustine and Aquinas taught ensoulment at 40 or 80 days because they BOTH unintentionally fell into the insidious trap of abortion lies. I cannot read their minds, I can only judge there results. They believed the lies of abortion from Satan or from another person (Aristotle?) or from their own misguided ideas. Little by little, starting small, almost unnoticeable, a barely discernable compromise on the sacred life of the unborn started with them by believing intellectual sources rather than the Bible which no Catholic would ever do in a million years. They taught vegetable, animal, then finally human. They said abortion was wrong but a baby was not a human soul until quickening. My point is they compromised on the absolute humanity of all the unborn at every stage unto horrible consequences. Can’t you see that is the slippery slope that has led to virtually worldwide abortion on demand. They did not understand what they were doing but they should have. In fact they were in conflict with two obvious Bible verses that proved a spiritual soul a conception….

Psalm 51:5 - “This verse is, both by Jewish and Christian, by ancient and later interpreters generally, and most justly, understood of what we call original sin” -https://biblehub.com/commentaries/benson/psalms/51.htm

James 2:26 - “The meaning here is the obvious one, that the body is animated or kept alive by the presence of the soul, and that when that is withdrawn, hope departs. The body has no life independent of the presence of the soul.” - https://biblehub.com/commentaries/barnes/james/2.htm

Intellectual sources (Supreme Court & Congress & President) say abortion is justified and legal. Some people prefer intellectual sources rather than Catholic and Biblical sources.

St. Thomas Aquinas never commentated on Aristotle's On the Generation of Animals, but he does refer to it in Summa Theologica I q. 118 a. 2 ("Whether the intellectual [human] soul is produced from the s*m*n?") arg./ad 2*:
_{*cited in fn. 1 of Hyacinthus-M. Hering, O.P., “De Tempore Animationis Foetus Humani,” Angelicum 28, no. 1 (1951): 18–29.}

Objection 2: Further, as shown above (q. 76 a. 3), the intellectual, sensitive, and nutritive souls are, in substance, one soul in [a] man. But the sensitive soul in man is generated from the s*m*n, as in other animals; wherefore the Philosopher says (De Gener. Animal. ii, 3) that the animal and the man are not made at the same time, but first of all the animal is made having a sensitive soul. Therefore also the intellectual soul is produced from the s*m*n.

Reply to Objection 2: Some say that the vital functions observed in the embryo are not from its soul, but from the soul of the mother; or from the formative power of the s*m*n. Both of these explanations are false; for vital functions such as feeling, nourishment, and growth cannot be from an extrinsic principle. Consequently it must be said that the soul is in the embryo; the nutritive soul from the beginning, then the sensitive, lastly the intellectual soul.

Therefore some say that in addition to the vegetative soul which existed first, another, namely the sensitive, soul supervenes; and in addition to this, again another, namely the intellectual soul. Thus there would be in man three souls of which one would be in potentiality to another. This has been disproved above (q. 76 a. 3 [on how there is only one soul in a man]).

St. Thomas gives and disproves two more theories, concluding with his own view:

We must therefore say that since the generation of one thing is the corruption* of another, it follows of necessity that both in men and in other animals, when a more perfect form supervenes the previous form is corrupted: yet so that the supervening form contains the perfection of the previous form, and something in addition. It is in this way that through many generations and corruptions we arrive at the ultimate substantial form, both in man and other animals. This indeed is apparent to the senses in animals generated from putrefaction. We conclude therefore that the intellectual soul is created by God at the end of human generation, and this soul is at the same time sensitive and nutritive, the pre-existing forms being corrupted.

^{*on the terms "generation" and "corruption", see St. Thomas's short work On the Principles of Nature}

St. Thomas doesn't give a timetable for how long the "pre-existing forms" exist.

^{A good overview of so-called "delayed hominization" is The Ashley Reader: Redeeming Reason ch. 20 "When Does a Human Person Begin to Exist?" (p. 329ff., PDF pp. 337ff.) by Benedict Ashley, O.P.}

Short answer

After reading the 2010 paper When Does Human Life Begin? Conception and Ensoulment which I exclusively used for this answer, it appears that they didn't use Ps 51:5 (which is popular among pro-life activists today) because they wanted to avoid the Traducianism view and opted for the Creationism view (see Complete Answer below).

Augustine and Aquinas then simply adopted Aristotle's view: 40 days for males, 80 days for females. It may not be as "primitive" as you think, if we understand why they chose the Creationism view, which has to do with separating LIFE (which plants and animals have) and SOUL (which in Christianity must be individually created by God because of its spiritual essence). Modern theologian Louis Berkhof also held the Creationism view but used other Bible verses (see Complete Answer below).

Another 2014 paper Traducianism? Creationism? What Has An Ancient Debate To Do With The Modern Debate over Abortion? has a more detailed discussion why Augustine and Aquinas chose Creationism.

Complete answer

The paper discusses 3 views of ensoulment and various religions's view on ensoulment, including a history of Christian position from the early church father until modern times. My answer is exclusively based on this paper.

Bible verses by pro-life activists

The paper starts by reviewing a few Bible verses often used by Christian pro-life activists, but which in the paper author's opinion have ambiguous meaning so not used in the history of Christianity until modern times:

• Ps 139:13: "you [God] created my inmost being; you knit me together in my mother's womb" (ambiguous because the intent is not scientific, but a metaphor that God knows David intimately)
• Jer 1:5: "Before I formed you in the womb I knew you, before you were born I set you apart." (ambiguous because it merely communicates God's sovereign plan of Jeremiah, similar to Eph 1:4 that all of God's elect were chosen "before the creation of the world.")

Pre-existentianism view

... Souls are preexistent entities who await bodies to enter. ... Historically, very few within Christian circles have held or taught this view, though the Church of Jesus Christ of Latter Day Saints adopted it in the 19th century and certain New Age groups ...

Traducianism view

"soul" is present in both the sperm and the egg when they unite. The combination forms a new "soul" automatically and immediately. Traducianism has been held by ... Tertullian (c.160-c.225) ... [who] wrote that "we allow that life begins with conception, because we contend that the soul also begins from conception; life taking its commencement at the same moment and place that the soul does." Clement of Alexandria presented a much more detailed description:

The embryo is a living thing; for that the soul entering into the womb after it has been by cleansing prepared for conception, and introduced by one of the angels who preside over generation, and who knows the time for conception, moves the woman to intercourse; and that, on the seed being deposited, the spirit, which is in the seed, is, so to speak, appropriated, and is thus assumed into conjunction in the process of formation.

This view is also held by Gregory of Nyssa (335-c.394) and Maximus the Confessor (c.580-662). The latter is based on the argument that Jesus was fully human at conception, thus possessing a spiritual soul from that instant. Because Jesus was fully human, every human being receives soul similarly (immediately) at conception.

Although this view best explains the transmission of original sin (since God is only indirectly party to the transmission of sin) this makes the parents to be the true creators of life and God only a (dis)interested observer, implying a deistic God except in the case of Adam and Eve.

Creationism view

... the "soul" is created and introduced into a fetus by God at a point of his choosing, either at the time of a fetus's first breath, as was the case with Adam in Gen 2:7, or when God in his soverignty knows that a fetus is not going to be spontaneously (meaning "naturally") or intentionally aborted.

The [20th century t]heologian Louis Berkhof [is] a staunch proponent [who] sees a marked distinction in the Bible between the body, which is taken from earth, and the soul, which is given by God. [He cited Gen 2:7, Eccl 12:7, and Heb 12:9.] ... Physical substance comes from physical origins, and spiritual essence from a spiritual source .... Creationism is the most biblically-based view, claiming that "it is more consistent with the prevailing representations of Scripture than Traducianism."

Berkhof is just one of the more recent representatives of a stream of thought that is rooted both in ancient Hebrew beliefs and in Aristotelian philosophy, a stream that is shared today by rabbinic Judaism and by much of Islam. Aristotle equated “life” and “soul,” but described different kinds of the latter: vegetative, sensitive, locomotive, and intellectual. “In general,” Aristotle believed, “soul is imparted to the body in stages as each part is formed, and the specific soul is not actually present until the form is complete.” This “completion of form” takes place on the fortieth day after conception for males, and on the eightieth day for females. Augustine of Hippo (354–430) was a proponent of this view, and Thomas Aquinas (1205–1274) adopted Aristotle’s schema practically in its entirety. Aquinas held that

the body was formed gradually through the power transmitted by the male seed but the spiritual soul was directly created by God when the body was ready to receive it. Thus the embryo was believed to live at first the life of a plant, then the life of a simple animal, and only after all its organs, including the brain, had been formed, was it given, by the direct and creative act of God, an immortal spiritual soul.

The Creationist views of Augustine and Aquinas were the norm in the Christian West from the early fifth century to the late nineteenth century. ...

The rest of the Christian section of the paper discusses the view of various Popes, John Calvin, Martin Luther, etc.