That same logic could apply to Steam if you don't have it installed and have Epic installed. You're basically describing first-movers advantage.
Personally I don't see that as a reason not to try others. Nor as a reason to give Steam a monopoly (on my game purchases).
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Steams memory footprint is generally lower. The social aspect of it actually works and can be disabled to free up resources (first mover I'll admit). And the UI is dramatically less offensive than the other "game launcher/stores"
Steam gives me the option to launch into my library instead of the store which makes it feel dramatically less "blah".
In comparison, just to play diablo IV I had to click off two popups in the battle.net launcher of "buy this".
When you add shortcuts to games on your desktop it bypasses the steamui entirely and just launches the DAMN GAME. (it still does do an update/validly check with steam if you are connected to the internet tho).
Try them and dislike them, that's the proper way to do it. You've tried the sushi, I'm not disagreeing with any of your real complaints that have substance. But I would try to convince someone to try sushi before making deciding it is bad. Deciding something is bad without trying isn't keeping an open mind.
Steam certainly isn’t faultless here but it’s less clunky and and adds value with features like shoring up of Windows’ abysmal controller support, which makes a big difference. If competitors want people to want to install their stores, they need to be treating the project with respect and hiring on a team of full time well compensated high quality engineers to build it. Be a VS Code, not an MS Teams.
So while being the default (or only) store is clearly valuable, it is not the only path to a dominant position. As a Steam user, I find myself groaning if a game is exclusive to Epic or Battle.net (or whatever it's called now), but don't mind if I have to buy a game from GOG or a dedicated site put up by the developer. So the actual complaint isn't that the game isn't on Steam, it's that I don't want to have to install and use some shitty game launcher. I don't want it on my computer. People felt like this about Steam initially too, and it certainly has its shitty parts still, but it actually adds sone value over a raw installer exe, unlike some other stores.
It's all about the user experience. Minimal steps between them and the product. Buy on Steam? Great, I have that installed and ready to go. Buy on your website? Fine, I can autofill the purchase form, get an exe/zip, and I'm set. Buy on Epic? I have to install their store... then make an account... then add payment info... then find and purchase the game (while being bombarded with ads and flashy shit in an unfamiliar store interface). I think I'll pass.