alt Hacker NewsProbably 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.
Lots of assumptions that could be totally wrong. Custom distros are not new operating system - majority of hw support is because linux supports it. It would probably be more work for Valve to support only their hw instead of keeping what linux/arch support.
I am also not sure why you think they wouldn't want to end up replacing Windows. Thats exactly what they've been trying to do since microsoft included their store in windows. That is more than a decade long plan in motion which already failed once but they are still at it.