Probably for the same reason they don't support every Arch package on the Steam Deck out-of-the-box; it breaks easily and it's not their job to fix it.

Additionally, I think Valve doesn't want to end up over-committed to replacing Windows. They can handle the storefront side and do a decent job with handling the runtime, but actually committing to a desktop alternative to Windows would be spreading their resources thin. It feels like a smart call to not jump into that arena if your hardware products don't need it.