The rumor/theory I have heard about Netflix is that increasing discoverability too much would allow people to see two negative traits of Netflix: How often things come and go from the platform (which other apps like Criterion Collection embrace), and just how limited their library actually is at a given time.

Scroll through recommendations. It looks like they have hundreds of great movies for you to watch! And yes, technically they do. But look how many times they try suggesting the same movies in different categories, inflating the view in a way to make the library seem bigger. One movie might show up "Because you liked comedy..." then "Because you watched <comedy movie>" then "Light-hearted movies".

TLDR money and masking their poor library quality.