I'll bother and explain why you're being stupid and not understanding the thing you yourself posted.

From the definition of factors of production on Wikipedia:

"In economics, factors of production, resources, or inputs are what is used in the production process to produce output—that is, goods and services.

Simply put, rent is paid at INPUT, for things like land, in order to produce OUTPUT, which are things like goods and services. What Steam provides is a SERVICE, an output. You don't pay economic rent on outputs, you pay economic rent for inputs. Steam's service being: marketing and distribution of games in place of others, plus integration with analytics and a bunch of other features.

The comparison you're making is the same as saying you're paying rent to your team of marketers and accountants...

You could make a point and argue that artists are paying economic rent for Adobe suite, and that game developers are paying rent for unreal engine fees. Without those things, which are inputs for production, neither artists or game developers would have a product at all. Steam only comes into play once the final product is already done. You don't need Steam before the game is a product at all, which corroborates that Steam is not economic rent, for it's not a payment made for an Input in order to produce an output.

Also, in what way is the marketplace for games fixed? It's not a finite resource. There's no finite number of how many stores there are out there, anyone can go and make their own client and store. There are games and developers that up to this day make their own standalone launchers.

Steams offers a service, the best one in the block. You don't want it? You're entirely free to go and figure it out yourself. No monopolistic behavior in sight.