In all likelihood, insufficient coaching and rehearsal is why it feels coached and rehearsed.

A perfect presentation, which has been extremely well practiced and rehearsed, comes off as natural. Because they havent practiced it enough (and probably because many of them are simply not very good presenters, unlike say Jobs) it falls into the uncanny valley and appears unnatural.

Yeah, you're on to something. I'd guess it's intense and precise coaching paired with people who don't do this sort of thing very often. It seemed to get a lot worse soon after they started trying to get a bunch more presenters involved, some years back. My guess is they didn't think the first couple of those were polished enough, so cranked up the prep, which, when applied to people who haven't developed outstanding presenter-talent, gives you a really fake and awkward effect. I think the somewhat rougher early ones were nicer, personally.

To their credit, I guess, most other events I've seen like this since everyone started trying to ape Apple's announcement style have the same problem, and usually worse.

An actor can only do so much with a terrible script.