As a gamer, Steam is about way more than mere distribution. It's a community: discussions, chat, friends, voice chat, screen share, remote play over the internet, cloud saves, sales, filters, betas, tags, mods, library management, matchmaking, etc. It's way better at those things than Epic Store, GOG, Microsoft or Apple or Google stores. Valve has been evolving it as a platform for decades and its competitors just aren't very good at all.
Even if you can get the game's renderer and UI to reach parity, it's hard to overcome the UX and network effects of Steam. And that sort of ecosystem fragmentation (alternative platforms often don't offer crossplay) is bad for gamers, not to mention having to implement different multiplayer pipelines makes lives harder for devs.
Gaming on the web might be nice for trashy flash like games, but for anything more, why would anyone waste time on that when there's much better curated experiences on Steam, or barring that, mobile app stores and Switch? There are already enough excellent indie games to last several lifetimes. Not gonna waste time exploring the dark corners of the web to try some amateur's experiment with webgl. If they're not on an established game platform by now, I could only assume the devs care about their philosophies more than end user experience, and it would be a subpar experience even if I got the graphics to load (big if).
Even something like GeForce Now (cloud rendered streaming games) use Steam and Steamworks APIs for the community and multiplayer APIs. It's therefore much better than competitors like Google Stadia (which is pretty much dead and has no community). GOG has a niche but only for single player games. For multiplayer it's either Steam or dead matchmaking.
Maybe a more interesting comparison is something like boardgamearena or boardtopia, which offer not just simple games but also the marketplace and matchmaking features of their own. They have established their own communities finding a niche (digital low fi board games) that Steam has neglected.