This is essentially boiling down to the logic of mutually assured destruction. A reasonable party would rather not see his country get turned into a glass crater because he launched a first strike. But there's always the danger of some nutjob Putin or Kim who decides that having another five years in office is more important to them than the lives of every man, woman and child on earth.

Of course, what do they care? If they lose, they won't live long enough to see it, while if they win, sure the world is wrecked, but hey, they survived and kept hold of power, and that's the really important thing.