Altars in today's Christian Churches

I think you have a bit of a loaded question, assuming that most church buildings don't contain altars unless you have some sort of non-anecdotal stat to prove it or give credence to it.

But as to why a local Protestant church wouldn't have an altar, I can say that those Christians would believe that God no longer dwells in altars or in things built by human hands, but instead dwells in his church, the children of God.

Paul had this to say to the Corinthians:

1 Cor. 3:16 Do you not know that you are God's temple and that God's Spirit dwells in you?

1 Cor 6:19 Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God?

Stephen, when interrogated by the High Priest, said the following:

Acts 7:45-50 After receiving the tabernacle, our ancestors under Joshua brought it with them when they took the land from the nations God drove out before them. It remained in the land until the time of David, who enjoyed God’s favor and asked that he might provide a dwelling place for the God of Jacob. But it was Solomon who built a house for him. However, the Most High does not live in houses made by human hands. As the prophet says: ‘Heaven is my throne, and the earth is my footstool. What kind of house will you build for me? says the Lord. Or where will my resting place be? Has not my hand made all these things?'

The Protestant answer (excluding high-church European Reformation 1.0 churches like Anglican and Lutheran) will be simply this: Because an altar is for sacrifice, and Jesus made the once for all sacrifice. So rather than offering a sacrifice ourselves on an "altar" we simply remember Jesus' sacrifice around a "table," the Lord's Table (thus Paul calls it in 1 Cor 10:21).

1 Cor 10:21 Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils.

Even the Catholic church, post Vatican II, is coming around to this view. You can see that instead of the old "high altars" they often tend to have much simpler tables. Instead of the priest standing ad orientem (toward the East, the back wall of the church where the high altar was, and hence away from the congregation) the priest stands in front of a simple wood table and faces the congregation while administering the Eucharist. (A traditional Catholic centerpiece for the old high altars, the "tabernacle" which contains the reserved host, which Catholics believe is the body of Christ, at times when the Mass is not being celebrated, and to which the priest would bow at several points, was often not clearly visible in this setup, although Roman Catholic Canon law prescribed

The tabernacle in which the blessed Eucharist is reserved should be sited in a distinguished place in a church or oratory, a place which is conspicuous, suitably adorned and conducive to prayer. [Canon 938 §2 of the Code of Canon Law, 1983])

It's one of the things that really riles the "traditionalist" Catholics up against the Novus Ordo mass. They perceive the Novus Ordo as favoring table-language and table-orientation, while the Tridentine mass (Traditional Latin Mass [TLM], also now called Extrordinary Form [EF]) favored the ad orientem (towards a high altar) orientation.

Actually, more Christian Churches have altars than not. They are the focal point for worship in Anglican, Catholic, Lutheran, and Orthodox congregations. In the Old Testament Hebrew worship, from which a number of essential elements of Christian worship derive, the Altar was a place of offering, and in the Hebrew tradition, this meant places for animal sacrifice--sin offerings, thank offerings, and other sacrifices as detailed in Mosaic Law. For those Christian churches which retain altars, they are still the focal point of offerings, not animal sacrifice, but rather offering of our selves and those things with which God has entrusted us. Nearly all Christians have dispensed with animal sacrifice, seeing instead the crucifixion as the perfect sacrifice, made once for all, and all time.

It is true that most Protestant traditions other than Anglican and Lutheran have replaced the altar with the communion table. The reason for this is that generally, those traditions, while still seeing the crucifixion as the one perfect offering, see the Last Supper, and the words of institution, as a sufficient remembrance of the perfect sacrifice.