nah is this case adding UWP apps to steam is totally on MS ... there is no problem to add everything else as "nonsteam game" to the library as everything you need to do is add and Exe and run through it and guess what UWP are launchable only through powershell link , tunning through exe does jackshit
You can't just add the exe for a lot of games. Many Origin, Uplay and EGS games will break if you do that. That is totally on Valve, as other launchers like Playnite have managed to accommodate these games. Meanwhile, to get games from these launchers working as they should on Steam you need something like
GloSC or
OSOL.
Recently I was experimenting with EGS free games to see how easy it would be to add them to Steam (which, other than philosophically disagreeing with exclusives, is the biggest barrier to me using their store), and it was a mixed bag. RiME and City of Brass seemed to work by just pointing Steam to the exe, but Fortnite and Dauntless required some (for novices) rather finnicky steps (including editing an .ini file) that could be made a lot easier on Valve's end.
Pretty much any modern Origin game won't work by just adding the exe. I discovered OSOL because I was looking for a way to add Mass Effect 3 and Titanfall 2 to my Steam Library and use it with the Steam Controller on my Shield TV (which I have connected via VirtualHere). In that case it's a lot more simple than EGS (as OSOL's default configuration is designed for Origin), but still requires some "backend" configuration, which is especially annoying if you are a Big Picture user.
The only time I've really had seamless adding of an exe to Steam is either DRM-free GOG/Humble/other games, or a game which has DRM but runs independently of any client (like an MMO like FFXIV or older versions of games bought from Telltale's store, before they stripped the DRM from them). The second you bring a different launcher into the equation, whether the game actually works becomes a crapshoot.
The reality is, just like Playnite detects my different launchers and finds my installed (and even uninstalled) games, Steam should be able to do the same. Just imagine being able to just see a list of all your installed UWP, Origin, Uplay, EGS and so on games, click a little tickbox, and have it appear in Steam without any tinkering? Valve is all about creating turnkey solutions where possible, and they have done an amazing job in so many areas, but how Steam handles non-Steam games - especially ones tied to a different launcher - leaves a lot to be desired.
I agree that UWP is different, and MS were deliberately trying to frustrate you doing anything outside of their store, but Valve could find a way to make it work. And even if it's not Valve's responsibility, where compatibility has been a problem in other areas they've taken it upon themselves to fix it. See Proton and controller support as two examples of this.