Also, all a nuclear bomb requires is enough fissionable material in a small enough space that the reaction is run away. I see no reason to suppose that a spot particularly rich in uranium (or other such materials) couldn't get squeezed hard enough to explode, or that a critical mass somehow formed by natural processes (particularly those which put it under very high pressure).
I suspect that the reason we haven't observed this happening is because most of the material is not present in concentrated form and because it decays over time.
But just so you know, people have accidentally assembled critical masses by hand. There's no special magic to it, other than getting a large enough quantity of suitable material in the first place (which is really, really hard). You can read some of the scary things that happened here:
They called it the "demon core" after a few people died working on it.