why don’t they give concrete examples of how human extinction is going to happen within five years?

Here’s one. AI is already very good at manipulating people via social media, and unfolding/combining proteins. It’s not hard to imagine an AI designing a virus that could wipe out humanity and manipulating some people to manufacture and release it. Hell, there’s already people working in AI cheerleading human extinction, so I don’t think the manipulation is even necessary. The main barrier is that it might be a while before AI can self-sustain and replicate without human help, assuming it would have an instinct for self-preservation. Once that’s solved, we’re toast.