Probably because you're expecting it and looking at a demo page. Put these voices behind a real video or advertisement and I would imagine most people wouldn't be able to tell that it's AI generated at all.
It'd be annoying to me whether it was AI or human. The faux-excitement and pseudo-bonhomie is grating. They should focus on how people actually talk, not on copying the vocal intonation of coked-up public radio presenters just back from a positive affirmation seminar.
