There are probably economies of scale, sure, which is part of why Epic can get away with a lower fee for now. But it's also just business. They know how much it costs without economies of scale, and that sets a floor.

https://youtube.com/watch?v=stxVBJem3Rs Here's a talk from a very old indie company, there's one section talking about how they existed pre-Steam (and other stores they use since Steam's not exclusive) and appreciate what it's enabled.