There are plenty of competing PC game online stores, it's just that they all suck monkey balls when you're not using Windows. Microsoft is currently using their old monopolist playbook and release Blizzard games to the fucking Microsoft Store and Game Pass and not a single 3rd party store.

And don't forget that the other publisher-owned storefronts like EA's and Ubisoft's are also still alive. They suck hard but they exist and apparently they do well enough to continue to be around.

Steam is the only PC games store that fights Microsoft's Windows monopoly. GOG Galaxy has been written using the Qt framework. Making a Linux version of an existing Qt application is relatively easy (at least compared to a full port). Do that, integrate umu-Launcher for Windows games, bundle everything up and release GOG Galaxy on Flathub. Boom, done. But they don't do that despite their massive pile of Witcher and Cyberpunk money.

So plenty of competition exists but if you happen to not be Windows-exclusive, everyone but Steam is bad.