Windows is bad for handheld gaming devices since it is not built to be used in such a way. There is a rumor that a handheld gaming version is on the way for Windows, but as of right now SteamOS is the only widely available mobile gaming OS available.
I mean, you’re not wrong, but I’m pretty sure Valve would allow other companies to download SteamOS for their own platform. You can already do it yourself for free.
I’m okay with that. I don’t want some program I don’t control having access to the kernel of my system.