The failures of other companies trying to emulate or capture that market do not a monopoly make. There is no real moat for Steam beyond customer loyalty and the fact that nearly every competitor sought to gain market through anti-consumer moves (exclusivity) rather than value-adds, and almost universally with shittier software to boot. There are a few notable counterexamples (GoG is a good store, value add, respectful of customers; but just didn't have the juice to establish itself beyond indie/abandonware games; Itch.io is doing fine in its niche).
Just because customers prefer a product does not mean it's got an unimpeachable moat.