Probably because Amazon is known more for their books than their movie/TV show selection.
Also: Prime gives you access to the lending library, but you can only lend one book a month.
Also: Prime gives you access to the lending library, but you can only lend one book a month.
I'm a Prime subscriber and I didn't know about this. It seems the lending library has 350K-500K books, not dissimilar to Kindle Unlimited. And like other commenters, it looks like both unlimited and the kindle lending library has pretty small intersection with my reading habits.