Because it’s open source and based on the Linux kernel. It’s owned by them but you can do what you want with it. You can’t with Windows.

So if a game works on the Steam OS, it works on pretty much any distro

I game with Steam on Linux, but I’m not using Steam OS

Also, that means that every effort made by Valve to improve compatibility is beneficial to everyone.

Edit: Also, even if it were closed source, I think it would still be good as it gives us alternatives to Windows. But