Have you ever even tried Heroic or Lutris as launchers rather than the store-specific launchers?!

They integrate with pretty much all big stores so you can see your entire games collection (not just the subset for a single store). Also they're way (WAY) more customisable and flexible in how you run you game, so for example most of my games running in Lutris run behind a proper sandbox (firejail) with networking disabled (something that the No DRM of GOG games also helps make possible, as to be sold via GOG games can't require a network connection to run).

Furter, the Open Source launchers don't care how you sourced some of your games - if you have it you can add it there and launch it, even if it's an old game with a CD installer or something you got from sailing the high seas.

I have both the Steam Store App and Lutris and whilst Steam has a slicker UI and deeper integration with many games from that Store (for stuff like Achievements), using Lutris is IMHO a better experience, especially when it comes to supporting my entire games collection, what I have access from the UI to tweak to get the more stubborn Windows games to run in Linux (you can easilly change actual launchers - including but no limited to Proton - as well as seriously customise how the game is run), overall privacy protection and digital safety (the whole sandboxing with networking disable thing) and, curiously, even because games in Lutris launch way faster since unlike Steam when I launch a game in Lutris I'm not constantly forced to wait for the store app to update before the actual game starts.

Things like Lutris and Heroic are done following an Open Source ethos (highly customizable, which also means you can get overwhelmed with options if you try and change anything from the defaults) and no trying to serve objectives which are store-specific (i.e. they're not at all trying to do what's good for that store, such as shoving adverts in front of you to buy more games or supporting high integration of specific games and a specific store).

Whilst in Windows I actually tended to avoid launchers (literally installed GOG Galaxy once, checked it out and then uninstalled it), after switching to Linux I have come around to really appreciate Lutris. I haven't got around to appreciate Steam all that much - it constantly gets in my way when all I want is to enjoy the game (generally because I often start the game from a desktop shortcut and have to wait for Steam to update before the game starts) and it's a bigger hassle to try and tweak stubborn Windows games to run in Linux via Steam than via Lutris and, of course, there's the whole "can't cut networking for it" thing.

Absolutelly, GOG's lack of support for Linux beyond just having Linux versions of some games available (which sometimes don't even work well) and the open REST API so that open launchers like Heroic and Lutris can talk to the store and fetch and install games directly, is a negative.

At the same time, the Steam app is also an inferior game launcher versus Lutris and Heroic, and I suspect so are all game store specific game launchers since they're designed first and foremost to benefit the store, whilst Open Source game launchers are designed to benefit the user.