I have several friends who used to work at Valve none of them hate the place, they still have friends there, etc. But they tell similar stories as to why things that normal companies do successfully are impossible at Valve. Perhaps it’s best summed up by something one friend said about her year and a half at Valve: “I first learned who my boss was on the day she fired me.”
Google tried this, notoriously dense grating and then firing basically all the managers at an all-hands. That didn’t work out well at all... And now they have over-steered in the opposite direction!
There is a strong sentiment with many gamers of just not wanting to use an alternative and it is basically a non starter for many other stores. Many complain about the very idea of not all of their games being in the same place.
This isn't necessarily anything monopolistic done on Valve's side. But it would be very hard for another store to make any meaningful impact regardless of how they are.
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.