Because why would we? Who is to say that whomever fills that power vacuum wouldn't be much worse.
The Kim family is enjoying the life.. They aren't interested in war with anybody. The people in the DPRK are responsible for their own freedom; and if the South Koreans feel any kinship with them, then, it's up to them to do what they will.
Tired of spilling American blood for people who will spit and trample on our flag.
The time to assassinate Hitler was before the war started. You could argue that Hitler's own military blunders allowed the Allies to win. Had he never invaded the USSR, he may have been able to avoid outright defeat. The Nazis and Soviets would have inevitably went to war; but changing tactics to bring about a stalemate that holds the 1941 position would have been the ideal strategy, imo.
From there, it's a race to the bomb. I still think the Americans would have won; and in fact, Germany might have suffered even greater catastrophic devastation had numerous atomic weapons been detonated over their cities.
So yeah, I'd say it's probably best we didn't assassinate Hitler.