Consider that they are rational and greedy. Like gangster barbarians, which they pretty much are, the only difference being they traded fur pelts for tailored suits. The smartest among them realizes that the project has failed, and that only hubris is keeping them going, hoping for something - anything - to change. But if having practical control over the obese, octogenarian U.S. president isn't enough to extract a favorable peace, then it's hard to see what else could possibly create a strategic win for Russia.

Why would they not fight to the last man, tank and artillery piece?

Because even now, life is worth living for the perverted gangster ghouls in charge. They haven't really lost anything personally significant. There is no opposition, and they can still live comfortably by Russian oligarch standards. Their biggest fear is each other.

You still need an army for token territorial defense and internal oppression. Russia is run by their intelligence services, after all. They're not fighting to the last man. They are already in negotiations with a position growing weaker by the day.