There's also the chance that some humans survive after the means of destruction (aka Nukes) are gone, and believe it, humans can be quite a pest, hard to eradicate, cause we can live in almost every ecosystem in the entire world. Nuclear war will mostly give everyone cancer and make everyone suffer major time.
Also, we have to remember big nukes have been mostly used as a DISPLAY of power (Hiroshima/Nagazaki), and not as a means of destruction. Why would a world leader eradicate humanity, if what they want is to rule humanity? Yes, when used, they bring destruction, but again, as a display of power, meaning "We can fuck you up so obey" and, you know, if Japan, of all nations, agreed, it must have been overwhelming, right?
There's also the fact that a nuclear weapon doesn't need to be a freaking earth shattering bomb. They can be very small and very targeted, and the tech has been improved to allow, instead of say 1 big bomb, a 100 smaller bombs that can hit any building in nation X, which is much better strategically speaking.
And yes, insane world leaders exist, and ones with nukes, but believe me when I say none of them can launch nukes by themselves. I doubt any of the coocoo idiots we have on power know how to use the advanced machinery to, you know, operate nuclear devices.
Nuclear war won't wipe out humanity, and probably won't wipe out one of us. It'll just make everyone suffer for years because their countries will have radioactive craters for years to come, where no living being could come near without having the worst cancer imaginable. So if suffering is your style, by all means wish for a nuclear war. You will suffer at least 10 times more, guaranteed.