Ukrainian girl in traditional costume.
Well because he doesn't believe that there's such a thing as Ukrainians at all.
And hence he doesn't believe that Ukraine exists.
A lot of current commentators will claim that this makes him a neo-Soviet, a claim for which there's more than a little truth, but in real ways, it makes him a neo-Romanov.
A little history.
Ukrainians and Russian stem from the same early Slavic people, a group of tribesmen who lived in central Eastern Europe. Indeed, modern Slavs are so closely related, genetically, that for the most part their DNA is of the same stock today. I.e., if you looked at a DNA sample of a Russian, and a DNA sample of a Croatian, you couldn't tell who was from where.[1] Only the Poles, who seem to have mixed more with their German neighbors, are genetically distinct.
Their origin is obscure, because the origins of all people are obscure. It's not really possible today to determine exactly where they stem from, but it seems likely it was probably from an area that today is in fact in Ukraine
Or maybe not. Some say it was in Pomerania.
Hmm. . .
Anyhow, originally all one people, they ultimately separated into East and West Slavs. The Ukrainians are East Slavs, along with the Russians, the Belarusians, and the Rusyns. You probably haven't heard of that last group, unless you are really up on things Slavic, and might not have heard of the Belarusians but for their communist government recently trying to flood Poland with Middle Easter refugees.
All told, there are 200,000,000 of them. . . which means that since World War Two their population has increased . . .well not hardly at all. Of the groups, the Russians are the largest. Rusyns are the smallest group, with there being just over 600,000 spread around Eastern Europe. Ukrainian are the second-largest group, and indeed Ukraine is one of the most populous country in Europe.
Anyhow, no matter what their origin, like all expansive people they spread around, and as that happened, they eventually had separate cultural developments. This is the same reason, for example, that the Germans and the Dutch aren't the same people even though they are linguistically and genetically very similar. And it's the same reason that the Swedes and the Germans aren't the same people as well, even though they ultimately are both Germanic peoples whose distant ancestors were the same folks.
All of this gets more complicated the closer people are culturally related to each other. Germans and Swedes may have all stemmed from the same people in some smokey village on the Steppes, before they started fleeing from the Slavs, but they clearly aren't the same people now. What about the Norwegians and the Swedes?
Well, as recently as the late 19th Century, that point was somewhat debated. Yes, they were distinct, but not that distinct. In Medieval times they were also distinct, but sufficiently close that they were often ruled by the same monarchs. Their languages are, moreover, cognate.
Another example might be the Irish and the Scots. The Scots are in fact descendant from Irish invaders into northern Britain in the 400s. Since then, however, 1600 years has passed, and they clearly aren't regarded as the same people.
The Ukrainians and the Russians are something like that. Ukraine might have been, or might not have been, the ancestral homeland of all the Slavs, but people and cultures changed as they moved about, and the Slavs really moved about, a lot.
As they did, the Russians became the most numerous and the most territorially aggressive, although occasionally the Poles would rival them for that title. In both instances, Ukraine was frequently their victim, as Russians claimed Ukraine was Russian and the Poles claimed it was Polish. All the while, the Ukrainians kept claiming that they were Ukrainian, and given the chance, as they occasionally were, they'd form their own nation. Usually that nation, however, has lived with the threat of Russian or Polish invasion, with both happening occasionally. The Poles threatened the then newly, and briefly, independent Ukraine as late as the 1920s, not all that long ago historically. And of course the Soviets reincorporated Ukraine into the Russian Empire in the 1920s, over the violent opposition of the Ukrainian state and then a Ukrainian guerilla movement.
Both reemerged during World War Two, making Ukraine's Second World War history complicated. In some regions, there was widespread collaboration with the Germans, which was a reaction to heavy Soviet repression and the mass starvation that had been imposed on the region due to Soviet occupation in the interwar period. In other regions, and more and more as the war went on, Ukrainians served in the Red Army or partisan groups. However, a partisan movement also emerged that fought for a free Ukraine, that being the Ukrainian Insurgent Army emerged during the German occupation to resist it in favor of an independent Ukraine, and to resist the Soviets for the same reason.
During its existence, the UPA fought the Poles, Germans and the Soviets, with the Soviets and the Poles being their primary opponents, but the Germans being real ones as well. Red Army mortality rates fighting the UPA after World War Two were higher than they were in Afghanistan, with some areas being particularly bitterly fought over, but without outside support, the Ukrainian guerrillas passed into history for the second time in the 20th Century, in this instance ceasing to exist probably around 1949, and in no instance later than the early 1950s. The organization did not wage a "clean" war by any means, taking genocidal actions against displaced Polish populations.
So, historically, the region has certainly fought for its independence, not always admirably, from any other power.
The Ukrainians have their own language, but it's cognate with Russian, showing how closely related the peoples are. The language shares some similarities with Polish, but they are not cognate, with the distinction being an odd one that has to do with listening to Polish in a studied manner in order to learn how to understand it. Ukrainians have their own branch of Orthodoxy, Ukrainian Orthodox, which just received autocephalous status from the Greek Patriarch, although a branch loyal to the Russian Orthodox Metropolitan exists as well. It also its own branch of Catholicism, that being the Ukrainian Greek Catholic Church (the Ruthenians, with that name being the ancient name for Ukrainians). Overall, over half of the Ukrainian people are members to some degree in one of the three Christian churches native to the region, with some being members of other Christian professions, and some Tartars retaining Islam.
The religious distribution of the country is revealing about the current situation, as is the linguistic one. On its margins, Ukrainian speakers bleed into Russian ones, with it being the case that the closer a person is to the eastern border with Russia, the more likely it is that a person speaks Russian. In terms of religion, most Ukrainian observant are Ukrainian Orthodox until a person approaches the Polish border, at which point Ruthenains dominate.
Ukrainians claim additionally culture distinctions beyond that, which no doubt exist, but which I'm not any sort of expert regarding. Architecturally, not too surprisingly, the further west a person goes in the country, the more its architecture resembles that of Austria. Supposedly, Russians are much more communal and gregarious than Ukrainians, and the legendary Russian association with alcohol supposedly does not carry over into Ukraine.
Like every region of the former USSR, there are a lot of Russians everywhere. The Soviet Union was oddly admirable and not in regard to its view of its imprisoned nationalities, in that the Communist did elevate to high position many who were not Russian in ethnicity, Stalin perhaps being the most famous example. At the same time, the USSR steadfastly engaged in a policy of Russification everywhere, exporting Russian populations to non-Russian regions, and conversely exporting wholesale some non-Russian populations elsewhere. In this context, it is not surprising that there are large Russian populations inside of Ukraine's borders, and it's more surprising that they are not evenly spread, being concentrated instead on the regions which actually border Russia.
What is going to occur is now, to a degree, anyone's guess, but Putin appears to be taking a page out of Hitler's book in regard to Czechoslovakia in 1939. The Donbas is Putin's Sudetenland, and the globe is already talking about concessions to Russia and its "legitimate" grievances. Hardly noticed is that the Russian population declined by 1,000,000 people last year, 600,000 due to COVID deaths, and the country is a mess.
There's no reason to believe that his appetite, in regard to Ukraine, or the former Russian Empire in general, stops at Donbas. The real question is whether the Western powers, here and in the Far East, are now willing to allow the globe to step back into the geopolitical atmosphere of the late 1930s.
Ukraine will provide the first test.
Footnotes
1. Ukrainians claim that this is not true, and that you can tell the difference between Ukranians and Russians by mere appearance, with the Russians being more Finnic.
Whether the DNA bears this out or not, it would actually make a little sense. As the Ukrainians are to the south of the Russians, their ancestors should not encountered what was the vast Finnic territory, originally, and its inhabitants the way the ancient ancestors of the Russians did. In contrast, the Ukrainians heavily encountered the Scandinavian Rus, and ironically the Kievian Rus gave Russia its name.