You’re failing to separate “Ukraine” the concept of a nation from “Ukraine” the government from “Ukraine” the group of people. In material terms, if life as part of the Russian Federation is comparable to life as an independent Ukraine (and Crimea was doing alright when there wasn’t much fighting), what does a war accomplish? Especially a war that the Ukrainian military was always going to lose. The end result is, as Yogthos said, incredible human suffering without even accomplishing the worthless task of keeping your preferred flag flying.

Now, I don’t actually agree that Russia wants to annex all of Ukraine. I think they want to break the back of the military and annex as much of Ukraine as votes to join them, but handling the colonial occupation of a country that wants to secede (like Ukraine had been doing with Donbas) is not in its interests. That said, even if they just wanted to take over the whole of Ukraine, a negotiated peace that wins concessions for human welfare would, in every respect, be a superior result to a losing war unless you’re a dog of the west and see damage to Russia as worth throwing generations into a meat grinder.

Edit: The reason why, to pick an example you probably are inclined towards, it was reasonable to fight desperately against the Nazis is twofold: One, the occupation represented a disastrous change for many, many inhabitants, such that few families would be untouched by the genocide (to say nothing of the national looting). Two, the Nazis were always likely to lose in the end because of their unstable model of operation, with many powerful enemies, meaning that one’s own hopeless personal resistance contributed to the broader anti-Nazi struggle that would indeed come to a successful conclusion.

I’ve already said in so many words that Russian occupation is unlike Nazi occupation, and Russia does not seem poised to lose as the Nazis were, it’s being careful about who it attacks and when, while continuing to cultivate stable alliances with other countries great and small in the imperial periphery and semi-periphery. The fight of the Ukrainian military both has nothing useful it could accomplish and no prospect of contributing to Russia’s downfall (nor is there much reason Russia should be taken down ahead of the western bloc). It’s pointless.