There are other game marketplaces out there, but they're bad.

This isn't like the Apple App store where it's the only option on the platform. In fact, they've competed with Microsoft's store on some things. It's not even like Amazon where they strong-arm people selling things on the platform. Amazon does things like forbid anybody who sells on Amazon from selling the item at a lower price anywhere, including on their own site. I don't think Steam has any requirements like that. Steam's store has a huge market share because people like using Steam. AFAIK, Steam doesn't even do exclusivity deals, which suck for the consumer but are pretty standard for games, except with their own (Valve) games, and those are rare.

Not only does Steam have a user-friendly library and a user-friendly store, if you launch a game you bought on steam but that is published by a company with a shitty launcher / store / library (EA, Ubisoft, Rockstar), Steam goes a long way to neuter the shittiness of that launcher / store / library.

Maybe a 30% cut is too big. I don't know. It would be great if someone tried to compete with Steam while keeping the consumer-friendly approach Steam has. Maybe competition would reduce that 30% to something lower. But, most of the other game stores I know of have much less consumer-friendly approaches. The only one that's at all similar that I know of is GOG, and I do occasionally use them, especially for old games.