Why in English do we say "Tsar of all (the) Russias"?

According to Wikipedia:

The emperor or empress of all the Russias or all Russia (Russian: Император Всероссийский, Императрица Всероссийская, romanized: Imperator Vserossiyskiy, Imperatritsa Vserossiyskaya, lit. 'all-Russian emperor, all-Russian empress'),[a] often titled tsar or tsarina/tsaritsa, was the monarch of the Russian Empire.

https://en.wikipedia.org/wiki/Emperor_of_Russia

The full title of the emperor in the 20th century (Art. 37 of the Fundamental Laws) was:

By the Grace of God, We, NN, Emperor and Autocrat of All the Russias, Moscow, Kiev, Vladimir, Novgorod; Tsar of Kazan, Tsar of Astrakhan, Tsar of Poland, Tsar of Siberia, Tsar of Chersonese Taurian, Tsar of Georgia,...

https://en.wikipedia.org/wiki/Emperor_of_Russia

So this article has examples of both the singular and the plural of Russia in the English language.

And quite possibly the history of the use of singular or the plural of Russia in English versions of the title, and possibly the Russian versions as well, might be more complex than any answer so far gives.

This source http://eurulers.altervista.org/russia.html For the titles of he Russian Emperors gives the singular form "Emperor & Autocrat of all Russia" in the English translations.

And it gives examples in Russian written with the Cyrillic alphabet. Those who can read it could tells if all the Russian versions use the singular form of Russia.

Another source of English language versions of the title would be in English translations of the titles of Russian Monarchs between 1721 and 1917 in peace treaties.

Before the Mongol invasion the words "Rus", and "Russian land" meant a neighborhood of Kyiv. For example, in the Primary Chronicle, a person traveling from Vladimir to Kiev was described as "going to Rus".

After the Mongol invasion, several territories had "Russia" in their names. At various times there were Red (Червона), Black, White and Little Russias. Since 16 century, the princes of Moscow began to claim all these territories as their "patrimony". And they started to call themselves the "Princes of all Russias" or "Tsars of all Russias", thus claiming the territories which they never owned.

In 17th century the full title included: "всея Великия и Малыя и Белыя России самодержец", which is translated as "of all Great, Little and White Russias autocrator...", for the complete title and its evolution consult Wikipedia.

Most of those territories belonged to the Great Duchy of Lithuania, and later to the state called Rzeczpospolita (which literally means "republic" but usually translated as "commonwealth". It was the result of union of Lithuania and Poland).

But Moscow princes insisted on this title for centuries, and there were even wars whose cause was "incorrect addressing" of a Moscow prince.

By the end of 18 century the Moscovites conquerred all these lands (and most of Poland) and destroyed the Rzeczpospolita. The title "Emperor" comes in use in the early 18 century, but inside the empire the Emperor was continued to be called the Tsar.

The Moscovites not only conquerred the territories but also appropriated the name. Until 18 century you do not find in the maps any "Russia" which contains Moscow. The state centered in Moscow was called Moscovia, Muscovia, Great duchy of Moscow, Moscow Tartaria, or simply Tartaria.

Instead you find a "Principality of Rus" with the center in Lviv (=Lwow, Lemberg, Leopolis), a part of Rzeczpospolita.

The name "Great Russia" to describe the part that included Moscow as different from other "Russias" was introduced in 18th century, and the word "Great" had originally has the same meaning as "Magna" in Magna Grecia which meant the Greek colonies in Italy.

Here is an illustration of what I wrote: Beauplan's map of 1630. Warning: North is in the bottom of the map! It shows Magni Ducatus Moscoviae Pars in the East (left side of the map), and Russiae Pars, to the West of Volyniae, with main city Leopolis (Lviv). Same on all other maps.

Moscow never was in any "Russia" until 18 century, when the name Russia was hijacked by Moscow rulers.

When Tzar Peter I. proclaimed the Russian Empire in 1721, this was connected to a state ideology that saw Russia as a triune nation, one nation with three comprising regional sub-nations (Триединый русский народ – narod literally means "people"). In todays terms they would be Russia, Ukraine and Belarus.

The first to popularize this concept seem to have been

• a treatise from 1674 commonly called the Kievan Synopsis and ascribed to Innokentiy Gizel
• Theophan Prokopovich, Archbishop of Novgorod 1725–1736 and advisor of Peter I.

Just like the idea of trinity in christianity, such a vague conception is a good basis for strife and diverging interpretations. Is a trinity ultimately one, three or inexplicable? Each of those will bear not only its own use of grammar, but its own state ideology. The answer "three" was given by the Russian imperialists and russophile Ukrainians, "one" was used by Russian nationalists and the Soviet Union (the latter stressing that multiple old nations had now united into one "soviet people"). Ukrainian nationalists would not subscribe to the concept at all.

Keep that in mind when reading the Wikipedia article mentioned above. Its sources deal a lot with Russian-Ukrainian relations, a subject that already imposes a way of looking at things. The corresponding German Wikipedia article goes so far as to claim the tzar was referred to by Prokopovich as Zar der Großen, Kleinen und Weißen Rus, "Tsar of the Great, Little and White Rus", citing an article by Ukrainian¹ philosopher S. V. Sinyakov. Here is the context in which he talks about the triunity:

...Реально украинцы жили как в разных государственнополитических образованиях, так и в различных цивилизационых системах: поствизантийской православной и римо-католической. Все это отразилось на психологии и характере украинцев обоих регионов. Трудно не согласится с мнением известного украинского историка П. П. Толочка, что "жители центральной, восточной и южной Украины имеют больше общего с населением сопредельных регионов России и Беларуси, чем со своими соотечественниками на западе Украины". Последние, в свою очередь, ближе к своим западным соседям. По существу, мы имеем две ментальности в рамках единого украинского этноса.

Идея объединения Украины и России не была навязана московским государством, а изначально являлась украинской. Она исходила от части казацкой старшины и была очень актуальна уже начиная с конца ХVI столетия и весь XVII век. Реализовал ее выдающийся украинский государственный деятель Богдан Хмельницкий. После присоединения восточной части Украины к Московскому царству появляется новая государственная идеология Российской империи. Духовной конституции становится "Синтаксис" Иннокентия Гизеля. Именно тогда впервые появился тезис о триедином русском народе и о самодержце "Великой, Малой и Белой Руси". Его придумал ректор Киево-Могилянской академии епископ Псковский и архиепископ Новгородский профессор, поет и оратор Феофан Прокопович. С тех же пор иерархию Русской православной церкви фактически захватывает украинская партия...

Позже в России из-за установления абсолютизма была проведена унификация языка, культуры попрусскому образу. Казаки были переведены в крестьянское сословие...В тоже время казацкая старшина...стали аристократией Российской империи...

Украинцы были не объектом, а субъектом, творцом Российской империи.

DeepL translation:

...In reality, Ukrainians lived in different state and political formations, as well as in different civilizational systems: post-Byzantine Orthodox and Roman Catholic. All of this was reflected in the psychology and character of Ukrainians in both regions. It is hard not to agree with the opinion of the famous Ukrainian historian P. P. Tolochka that "inhabitants of central, eastern and southern Ukraine have more in common with the population of neighboring regions of Russia and Belarus than with their compatriots in the western Ukraine". The latter, in turn, are closer to their western neighbours. Essentially, we have two mentalities within a single Ukrainian ethnos.

The idea of the unification of Ukraine and Russia was not imposed by the Moscow state, but was originally Ukrainian. It originated from some Cossack officers and was very complete already from the end of the XVI century and during the XVII century. It was implemented by an outstanding Ukrainian statesman, Bohdan Khmelnitsky. After annexation of the eastern part of Ukraine to the Moscow Empire, a new state ideology of the Russian Empire appeared. Innokenty Gizel's "Syntax" becomes its spiritual constitution. It was then that the thesis of the triune Russian people and of the autocrat of "Great, Little and White Rus" first appeared. It was invented by the Rector of the Kiev-Mohyla Academy, Bishop of Pskov and Archbishop of Novgorod professor, poet, and orator Theophanes Prokopovich. Since then the hierarchy of the Russian Orthodox Church has effectively been taken over by the Ukrainian party...

Later, due to the establishment of absolutism in Russia, the language and culture were unified in Russian fashion. The Cossacks were relegated to a peasant estate...At the same time, Cossack yeomanry...became aristocracy of the Russian Empire...

Ukrainians were not the object, but the subject, the creator of the Russian Empire.

The Fundamental Law of Imperial Russia gave the (start of the) title of the emperor as "Император и Самодержец", commonly translated as "Emperor and Autocrat", the second being the term used by Sinyakov. This exposes the Wikipedia article as citing wrongly. Nonetheless, I think the reference is useful by giving a glimpse into an ideology that would use the plural form "all the Russias".

Edit: @Alex in his answer has pointed to the Russian Wikipedia page about sovereign's titles. That one lists the start of title used between 1667 and 1721 on the Grand Seals as

Пресветлейший и державнейший великий государь и великий князь Петр Алексеевич всея Великия и Малыя и Белыя России самодержец;...

Most Reverend and Sovereign Grand Duke Peter Alexeevich of All the Great, Small and White Russia, Autocrat;...

Its source is an apparently well-known 19th century book on heraldry by Alexander Lakier.

¹ I was not able to identify him clearly. The 2011 article is from a Ukrainian journal and says Sinyakov teaches at the Kyiv National Transport University, but I can't find him on their website. Also, DeepL identified the language of the article as Russian.