Yeah, I really can't figure out why valve isn't working harder on making SteamOS available to other handheld PCs.

The Steam Deck is sold at a loss, but all the benefits Valve gets from the Steam Deck have to do with it's OS. It's both clearing a path for non windows gaming PCs (it's dangerous to let a single rival company like Microsoft completely control your market), and while the deck allows you to install non-steam games, it's much easier to stick to Steam games.

So it would seem to me that putting SteamOS on other devices would be a unanimous victory for Valve, getting all the benefits of the Deck without losing money up-front by subsidizing the hardware.