Sure, and that's because:
1. Nobody bothered competing with them for years
2. Those that did eventually compete didn't get anywhere near feature parity
I think EGS and GOG could get most of Steam's features with 2-3 years of solid development effort, but instead EGS whines about Steam having unfair market share and GOG just refuses to innovate on their client.
That's not Valve's fault, what is in their control is whether they use their market position to kill off competitors, and they don't do that.