Mainly because they’re kinda useless in modern warfare.

And the big boys with the nukes will sit on you hard.

Yeah, in the same sense that murder is banned. The ban gives you an excuse to punish people who violate the ban, but the ban doesn't stop people from doing it.

And if one country keeps their nuclear program, who's going to do the punishing? Sanctions don't work. Saying you're going to ban nukes is pointless dreaming. Why stop there? Why not endeavor to ban wars as well? Just wave your magic wand and ban all the bad things out of existence.