I think that writing in general is just way more subject to being diluted and going bad than other aspects of film/tv-making, mainly because it's harder to judge good writing from bad for most people.

So half the writing team get swapped out for nepo cases because the producers can't tell the difference. Half the script ends up subject to the demands of said producers which makes things worse because they think they're better writers than the people they hired to write. The fact that there's a "writing team" at all prevents a strong authorial vision from existing - this isn't a huge problem in episodic TV, since each episode can kind of stand on its own, but for modern streaming shows and movies it's a big one.