Russia has been unable to capture all of the "4 Donbass regions", so Ukrainians would have to give them territory they could not capture in war, which means they would have to lose their frontline defenses. They would have to disarm, so would not be able to prepare new frontline defenses. That would leave them open for invasion... EASY INVASION.
And what would the Ukrainians get in return for leaving themselves open to invasion? A 30 day truce. If the Russians keep their word, they would be allowed to start attacking after 30 days.