But that's the issue. The competitors are fine. They aren't significantly better though. The only one with a compelling USP is GOG with their "no drm, download the installer, own the game even after we go under" pitch. Everyone else is just a steam clone with some exclusives and freebies. Without a compelling advantage network effect makes Steam the clear winner. It's where your other games are and it's where your friends are.
But that only holds true while Valve doesn't screw up. Their competitors can't be much better than Steam, but Steam could absolutely make horrible decisions that cause people to leave. But they don't. Their organizational inability to make decisive action without wider support has lead to an incredibly stable, predictable platform.