The iTunes movie store launched 20 years ago. It’s far from perfect but it is essentially steam for movies. Sadly it’s been de-emphasised over time. But it is still there and was pretty good for a while.

You can buy at several places that interop with each other—iTunes and Amazon are the two biggest. They don’t have literally every movie, but they have most that most people want to watch. https://moviesanywhere.com/participants

Cost ranges from $5-30. Fewer dirt cheap sales than Steam, but the standard price point at launch is lower, in exchange.

(Having to explain “buying movies” makes me feel old!)

As someone already mentioned. Steam for movies already exists (iTunes, also Amazon’s offering). The problem seems to be that hardly anyone wants to actually own a movie anymore. There are places where the ownership model seems to still be thriving (books), but for video and audio, ownership (vs. streaming or renting) is largely dead.

Steam had John Wick on it at one point

People are saying that you can buy movies online, but I think they're missing the key point of putting lots of movies on massive discounts and promoting the movies that are currently discounted. Like sure you can buy basically any movie on Amazon or Apple's store or wherever, but I know that wherever I go, it's going to cost $4 to rent a movie, except every once in a while when it's on sale and I get it for $3, and buying it is going to be some higher amount that is almost certainly not worth it. When steam has sales, I might browse and buy quite a few games that I'm not gonna play right away. Or I buy things in bundles because it just seems like such a good deal. If movies were usually $10 to buy, but the Amazon store had a very visible section of movies that were $5 or less, but for a limited time, I'd be way more likely to buy multiple movies that I'm not intending to watch right now.