U-crane is the elites laundromat.

It must be protected at all costs. As long as it doesn't get the elites killed.
Plus, the T.H.E.Y. want to start another world war, so they can mess with the worlds financial system without huge blowback (like they did last time with Brenten Woods)

Putin knows this and has stated it in his goals for the war with U-crane.
Thus he is fighting a slow, boring war

While Zelenski is not allowed to do any peace agreements.