It seems to me like you're discounting real and present dangers in favor of one that's more speculative. The reason why climate change and nuclear disasters (I did not limit it to nuclear weapons, but am referring also to events like Chernobyl and Fukushima, which have poisoned significant regions of the planet for millennia to come) are real and present is because a) climate change is already under way and causing massive demographic shifts; and b) nuclear technology is already in a state to destroy large parts of the species and render much of the planet uninhabitable. We are one button click away from a bomb going off at any time, and there are lots of crazy fingers in the world itching for a button.

Even nuclear meltdowns aren't' enough to literally destroy the planet. In the worst case they might kill a few thousand people and cause billions in economic damage. Which sounds like a lot, but on the scale of the whole planet is almost nothing.

All of them are real dangers, and present dangers at that. But they still are not existential dangers. Even in the worst case humanity wouldn't go extinct. It's unlikely even modern civilization would collapse, just a few countries.

AI is one of the few things that can literally wipe out humanity, or even the entire universe. So I think it's worth special consideration. Personally I also believe it's far more likely as well. And people are already doing things about climate change and nuclear proliferation. Until very recently, there were only a few people working on AI risk, and it's still nowhere near enough.