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.
And the result is that users overwhelmingly prefer to use Steam, with alternatives largely relegated to at best grudging acceptance for those games that require alternative launchers. Since companies are reluctant to post numbers, it's hard to tell what the exact situation other than "Steam is well over 50% of the market", but the next largest is probably GoG, especially if you exclude self-publishing from statistics (if you include it, the popularity of Fortnite might push Epic Game Store into second place). And note that GoG is pretty much the only store that offers users a specific value proposition to use them over Steam: GoG is DRM-free (better publisher/distributor split is a value proposition for developers, not users).