I feel like they've earned it because they've put in the most work. They are the best in the game because they make the user experience the best there is. EA, Ubisoft, and Microsoft have/had their own storefronts or launchers but they are clunky and unpleasant to deal with and the only benefits they had were exclusives. They've never put any effort into user experience and were mainly doing it to make themselves more money and it definitely showed. The only one that's ever been a real competitor is Epic Launcher. And while it has gotten better over the years, the user experience is still not anywhere near Steam. And even now the Epic Launcher is still unpleasant to deal with in a lot of cases unless you just use it to play Fortnite.
With Steam everything just works and is basically seamless. Not only that, before Steam the modding community for most games had an immense learning curve and most people just avoided it save for Minecraft. And as far as I can tell you can't even mod games you buy on the Microsoft Store because their file structure is atrocious.
The only storefront I wish was better/more popular was GOG. It's not bad and has a lot of benefits (Like no DRM and offline installers), but Steam just makes everything so easy it's hard not to get stuck with them once you've started.
