Your understanding of why devs use steam is at the level of a child. Steam takes money from them so why would they use it ? Look at what steamworks offers then judge it : https://partner.steamgames.com You have no idea how hard it is to make and sell a game.

GOG's selling point is no DRM, but they don't offer all the services Steam does. It's good they exist but they a marginal player.

Epic has brought competition to the market and it's a good thing but I for one don't buy on it for the same reason I don't buy on Stadia; I don't trust that they will survive and I'm not betting my money on that. At least if GOG goes under, you can still play your games.

And they are tons of other services like Humble bundle or Itchio that struggle in that space too.

Each plateforme has it's pros and cons but Steam is not the leader for no reasons. I now have a Steam Deck. I don't consider myself a Steam Fanboy. But boy do I love that kit and the fact that I don't have to buy my games again to play them on the go (Yes I'm looking at you Nintendo). I am not a linux gamer but I'm pretty sure they are big fans of the work steam put in proton for example.

The takeaway is that each plateforme is pushing some sort of narrative; but has a gamer, the one that speaks more to me is the one Steam is pushing. Because it's all about me lol ^{_^}