Maybe because many people still use libraries (which don't share data) so there's no good database of reading habits the way there is for streaming movies. (Which recs seem to work much better for)
Amazon, meanwhile, has an incentive to show whatever ads pay the most. In this case, it's something of a winner's curse situation -- the ads that pay the most are often the ones that overestimate the value of an ad placement, not the one who correctly estimates the value of an ad placement. And if the ads don't generate the expected revenue, as long as Amazon can find more suckers, there's never a feedback loop occurring where Amazon's poor ebook ad placement ends up aligning with anything they care about. So I get ads for a lot of self-published trash (either in the Get Rich Quick With Bitcoin variety or the modern day romance novel sort), Amazon gets money, and most of these authors need a very low conversion rate so what do they care.
They never, ever admit that they don't have what I'm seeking and then suggest alternatives.
I get it free from Tmobile but hardly use it.
Also, there are a lot of independent movies and TV shows on NetFlix. And they suck. Horrible acting and plot but the cover is sometimes good enough to trick you into wasting time taking a look.
Statistically, after viewing a great movie the next one will be rated as less satisfying. We build up our expectations to a level that is not in sync with reality. This psychological insight was used at one of the early Netflix challenges by one of the top contestants.