Other features steam develops for you with that 30% cut: Multiplayer friends list apis, cloud save apis and space, wide open VR apis (that get turned into Unreal Engine apis, game streaming, voice chat (though it's terrible by today's standards), workshop (modding and UGC) apis storage and management, Free keys to give out on other platforms which actually decreases that 30% cut depending on how much you use that functionality, built in 'markets' for in game items, steam remote play apis and functionality, remote play together api (streaming gamepad stuff over networks without needing any crazy configuration or special programs), the new input system which is just incredible and can basically eliminate any work an individual game developer has to do to support powerful input tools and accessibility, free selling games on linux with very little dev work needed to support it and way less demanding bugs from linux users, built in customizable (but purposely bad) DRM if you only care for a minimal implementation, etc

Gamers prefer Steam over EGS, and the reason they prefer it isn't just because 'it's a nicer store front.' It's a whole platform thing. I would question if EGS wanted the same 30% cut as valve gets. I would question Gamestop taking a 30% cut. Not every game benefits from all these things. And top of all that, it's just frankly where PC gamers are, so theres a ton of built in marketing.

a shopping cart and ability to purchase more than 1 game at a time (didn't know that was a feature, but apparently it is). And the store page has all sorts of stuff like ratings, reviews. They host forums, guides, achievements, cloud saves, multiple versions of the game at once with beta channel access, screenshots, remote play, extras like Proton support, a friends list that will show you when other people are playing a game (advertising). The 30% developer fee makes a lot more sense if you consider that steam is much more than a game store. While I won't argue loot boxes, and I dont know enough about mod/skin communities.

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