Why do unaligned AI not have any survivors?
As a "highly intelligent system", we have a long history of extincting animal species, and we're well on track to eventually extinct most of them, despite it's highly likely this will make Earth uninhabitable for us, ending with humanity dying off too.
Why do you think AI can't casually extinct us, because it doesn't need us (or thinks it doesn't), and we're just in the way of whatever it is that it wants to do?
Well, we care for various reasons, major one being our own survival and comfort. Given what that means in practice, we'd be better off dead than having an AI care about us like we care about animals and plants.