Because MAD only works if both state actors are rational (whereas a rogue/terrorist state would likely not be)

While relying on MAD and nukes isn't ideal, it seems to work overall. So if we accept that it's necessary, I think it makes sense to limit nuclear weapons to nations that can make and secure them - that isn't something you do in a garage, it takes a serious, organized effort.

Something you'd never expect a bunch of nutbars to manage as they're busy tearing apart their own back yard over ideology.

But then you come to the issue of failed states. Who guards the nukes when a society destabilizes?

I try not to think about it because I like sleeping at night.