I'm confused, who works on Proton is it Valve or CodeWeavers?
I don't see valve employees in there: https://github.com/ValveSoftware/Proton/graphs/contributors
I don't see valve employees in there: https://github.com/ValveSoftware/Proton/graphs/contributors
(Disclaimer: I worked at a contractor on the Steam Deck for a while, working on the desktop mode.)
All I can offer you is speculation: Maybe just something no one's gotten around to yet? Also, I think desktop mode and its use cases are probably a lot more popular than anyone expected.
https://www.theverge.com/23499215/valve-steam-deck-interview...