They did that 10 years ago. Right around the time they added the video stuff. Didn't work out. https://www.rockpapershotgun.com/steam-stepping-back-from-selling-films

Around 50 are still available if you know how to look for them: https://store.steampowered.com/search/?sort_by=Released_DESC&category1=992

Watch Kung Fury if you want to test it. I loved the cheese: https://store.steampowered.com/app/374570/Kung_Fury/

I don't think Steam's business model works well for Movies/TV. Besides delivering the game files after your initial purchase Steam also continues to host and deliver update files for games over time, as well as lots of extras like syncing game saves, the workshop for mods, etc. I like having a centralized service that offers these features and acts a launcher for games because it's very convenient. These features are a huge value add that makes the service very attractive over piracy.

But for Movies/TV the main thing I want is the ability to watch the content, at a high quality, on whatever device I want, whenever I want to watch it. Theoretically this shouldn't be to hard, but with the way all the rights work it's effectively impossible for any streaming service to offer this. Content gets removed all the time, it's spread across a ton of different services that all offer a different experience. In a vain attempt to thwart pirates it's a pain in the ass to watch content offline so it's unreliable at best.

The only way to get the experience I want with Movies/TV is to pirate the content.