What avenues of escalation are left to Putin? 80% of his army is tied up in Ukraine, the rest is needed for border patrol. All russian men that would join the army voluntarily have already joined, so a big mobilization would see the support for the war plummet. And after that, all that is left is the big N word. And there are plenty of reasons why Putin doesn’t want to use those.
On the other hand, letting Ukraine lose will lead to much more instability in the future.
Edit: above comment accused ‘lefties’ of wanting WW3.