I don’t see why Valve wouldn’t try to support lots of hardware. Small time outfits like CachyOS can do it, why wouldn’t they? I think their motive with the Steam Box is hardware sales. It seems like they’re trying to shift the gaming ecosystem away from Windows.

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.