Note that if you're an indie developer and you want to make money, you pretty much are going to use Steam. The alternative is if someone like Epic sponsors development or something. But yeah, you can release on stores like itch. Nobody will buy it, but you can do it.

This can lead to a little bit of a chicken-and-egg problem. Ideally, devs will put up both a Steam version and an Itch/GOG version. But in practice they often don't; you sell on the store that brings in the vast majority of revenue. And then pretty soon your mods start getting managed through Steam Workshop, your input starts getting managed through Steam Input, your achievements/multiplayer starts getting managed though Steam's cloud. Because frankly, players want that. Steam players want to use Steam Workshop, and they don't really care if that means the mods are harder to use outside of Steam.

None of these services can be separated easily from the Steam platform. You can't pay Valve some money and integrate seamless workshop support into your game when it's booted up from GOG.

So users may try to avoid Steam for a while (I generally do), but you can only really do that if you're willing to skip games (still waiting on Spelunky 2, Ultrakill, Cruelty Squad, etc...). GOG and Itch are great and some devs are genuinely great about supporting multiple storefronts, but it's always going to be a subset. So eventually users get tired and just buy everything from Steam. I mean, if you buy from Steam you get input profiles set up out of the box, you get cached shaders, and importantly you never need to wonder whether or not a game is coming to the storefront you use. Never. And if a game ever doesn't come to Steam (hello Epic Games) you can have an existential meltdown and say that the devs are ruining PC gaming.

And as a consequence, if you're an indie developer, you are releasing on Steam. Because it's really difficult for users to avoid Steam because that's where the games are, and as a developer it's really difficult to avoid Steam because that's where the users are, and we go round and round until a nontrivial portion of PC players believe that PC gaming effectively means Steam, and anything that's not on Steam might as well just not exist.

You kind of saw this with the Epic Games fights around exclusives. I'm no friend of DRM and I'm no friend of exclusives, but not being able to get a game because it's not on the storefront I want to use is extremely common for me, and it was extremely weird seeing Steam users act like this was the end of the world when it happened to them. It really cued me in to how much power that Steam has, to the point where people basically treat it as a default platform. On some level it is on devs like me to support multiple storefronts, but people need to recognize that there's really not a lot of incentive to do so beyond trying to make the ecosystem healthy. Breaking Steam's stranglehold over the PC market would need to be a coordinated effort from both developers and players, it's not something developers can do on their own.