Why not have some platform that lets users to actually buy the movie and download/stream it as a file at full quality on their device?

Renting is sucky compared to just buying things, you could watch any movie you want and have unlimited access to it instead of juggling 5 subscriptions and get frustrated with shitty products. And publishers that make actual good movies that people want to watch would be rewarded

Probably owing to the legal/licensing constraints other users brought up.

I don't want to buy movies, 99% of the time. Renting is cheaper, and I don't care to rewatch most things.

Games we used to rent those too, before they got to be 50-100 hours long. For several reasons it's no longer practical.