Well, that’s not entirely true is it. This territory is not known to have been a place missiles. Plus, Ukraine will not be able to take enough land to even make it a buffer zone for artillery, let alone missiles.
This land is a liability. You need many troops to defend it that could be somewhere else. That’s a fact. The is only a question Wether the pros outweigh the cons.
Of course we don’t know more than Ukrainian commands. We are speculating and talking. But they make mistakes as well. In war there is a game of probabilities and risks.