The USSR and the West were essentially racing to partition the axis. If either one had made peace it would have assured the other achieved total regional (and therefore probably global) dominance. So even if Germany had offered some kind of deal in say 1944, like restoring the French government and making peace with the non-Soviet allies, that would have just allowed Stalin to control the fate of Europe. Also, it would have risked Hitler staying in power should the Soviets also waver.