Then why are publishers and customers so loyal then? There has been attempt after attempt to create a competitor but they all fall short. Steam offers so so so much in comparison to the competition it's not even funny. The 30% is justified.
I'd argue that Valve does more than just take 30% as a middle man. Between Steam Input, Proton, the beta built in recording system, the Forums for every game, community system and the marketplace, having your game on Steam is a massive value generator for the consumer and by extension developers.
30% might not be what the industry standard should be, but Valve isn't just providing a standard digital distribution service.
Steam is the best, and we're lucky that Steam is the one that won rather than another. Which could definitely have happened because once one of them is in place it's extremely hard to change.
So the situation is good for gamers, but from an economic point of view it's bad.