Russia repeatedly made peace talk attempts early on. Western powers that actually call those shots rebuffed them. Boris Johnson himself intervened, allegedly.
The answer to the real question, which is why Russia isn’t unilaterally ending the war, is that its objectives have not been met and/or the status quo is acceptable to them. The former is the exact same as saying why Russia invaded in the first place.
So why do Western powers want this was to go to the last Ukrainian? NATO military tactics that assume air dominance without the air dominance. Zero expectation of a win, despite the propaganda.