Sure, many games are tied to various Steam services, but that's by the choice of the games developer. Steam offers various built in services that game devs can choose to use if they want. It's not like it's some kind of requirement.

You might as well complain that game devs use Windows binaries, locking their games to only run on Windows. Sure, I prefer it when they target other platforms, but that's 1000% not Microsoft's fault that the dev chose to dev for their platform. I'm not mad at Microsoft for so many games being Windows only. I'm mad at the devs.

And games that build themselves around Steam services are of course going to be tied to Steam. That's a choice the devs made. If they wanted their game to run without needing the Steam client, they trivially could have built it that way. They just would have had to either reimplement all those Steam features themselves, or done without.

And if people want those Steam features, every store client who wants to run those games would have to implement those features in an interoperable way. It's easy to say "have interoperability between clients," but that's glossing over the potentially thousands of dev hours required to implement all of the features needed. And that's assuming they could all agree on a spec.

And to your final point about being open source. First, it gives very "any musician who gets paid is a sellout" energy. But more than that, it doesn't actually solve the problem you have. Even if Steam open sourced their tooling, that doesn't mean other players in the space could integrate it. Steam has grown organically for the past 30yrs, and trying to extricate the deep inner bits and then graft them on to your own solution isn't as easy as it sounds.