The problem is that it's obvious by now that Russia can't match Nato in this fight with conventional weapons. So from their perspective they probably don't see any other option at this point. They can't win a conventional war with Nato that's certain. They probably can't win a nuclear war with Nato either but there is at least some degree of uncertainty in that. It's a choice between zero percent chance of winning and a non zero chance of winning.

I am of course presuming all diplomatic avenues are off the table at this point.