Price discrimination or price segmentation is the technical name.
If you are selling an identical good which has near zero marginal cost to reproduce, then the way to maximize your profit is to sell it to each person for the maximum they are willing to pay.
Ideally, you want to sell (or rent) the same movie or tv show or song to someone willing to pay $5 for $5, $10 for $10, and $1 for $1.
In practice, it is logistically infeasible to target each and every person’s maximum price, but you can try to target populations as a whole. For example, grocery stores with no discount to people who are willing to pay more, versus giving out paper coupons or online coupons to those willing to spend time to save money.
In media’s case, I am assuming that the media sellers are betting the people willing to buy online are willing to pay more, on average, than people willing to go through all the trouble of renting a physical disk.
At least in my case, it would ring true. If I really wanted to see something, I would not care about paying $5 in the moment on my TV and start watching in seconds, rather than remembering to get and dealing with a disc from a Redbox kiosk for $1. But there are people who would want to save the $4, and so the content sellers are able to get $5 from me and $1 from the person using Redbox (although they are also losing sales from people not willing to buy at $5 online, and not willing to pay $1 at a Redbox, but the bet is that population is smaller than the total of the other populations).
Compare the current digital rental prices to taking yourself to the theater. While the digital rental rate is high, it is less than one ticket for admission. If you buy concessions, it only goes up. If you take someone else, it gets higher. That one digital rental starts to look less steep from this vantage point. That being said, I still don't do the digital rental.
Note you're not just paying the marginal cost, you're also paying for the streaming infrastructure they invested in setting up.
Edit: very old numbers can be found here. https://arstechnica.com/uncategorized/2006/03/6400-2/
The cost to build out the infrastructure to stream movies is tremendous.
It'd also help a lot if I didn't need a different "app" for every store, with its own player UI. Learning how to use yet another designer's cute "experience" just to do the same thing I used to do with a few buttons on the front of a VCR that were the same for every single movie, isn't my idea of fun.