That's because AIs make mistakes no human would. If you're documenting a function, you might misspell something (or orthographically spell something that should be misspelled), or confuse two functions with similar names; you won't reference a function or feature that doesn't and has never existed.
That does sometimes exist in human docs. I've stumbled on "aspirational" features in docs that never really existed but were more just intended to exist.
To me the mistakes are bad, but it's also that the docs tend to be pretty low quality IMO. LLMs tend to like to write novels where a sentence would have been better.
Listicles and those sorts of things were pretty pointless, but they didn't beat around the bush like LLMs do. I really don't exaggerate when I say I've never seen a human write like that. Like, the moment I start reading AI responses I feel the urge to skip ahead to get to the point. I probably read at most like 10% of the words they spit out.