The same reason the Russians do. Not sure you explained this in your piece. A gray zone can be taken over by either side but at the cost of attrition (soldiers dying going in and holding). As you point out, no sense for Ukraine to go in because the next stop is Russian territory.

There is sense for Russia to go in IF it's going to continue. But it can't, as you point out 50,000 not enough. So the move was purely political. A bullying move which succeeded because Western press basically said Russia is about to take Kharkiv and Ukraine should give up.

Eh, what can you do ;) Those Russians died for nothing but politics. In a few weeks that lame effort will be forgotten.