1. Because giving one party everything they’re asking for at gunpoint isn’t really a “deal,” that’s just them winning.
2. Because part of the concern is that there’s no particular guarantee, if we have a peace deal, that the peace will continue after the deal. Both iterations of Russia’s original invasion were accompanied by strident denials that they were invading, because that was crazy Western propaganda, and nobody’s going to invade Ukraine. A person who’s willing lie to you about the present and past, is even more likely to lie to you about the future.

Edit: Actually, even more telling: One of Russia’s key demands, apparently, is that it is ABSOLUTELY OFF THE TABLE that Ukraine get security guarantees from anyone, now or ever in the future. In the mode of abusers everywhere, the one thing that is an absolute atrocity which will cause them to go ballistic is if they are threatened with consequences if they don’t adhere to the terms of the deal. The deal they are swearing they will, of course, how could you even think otherwise, adhere to.