This is changing. Valve is actively working with hardware manufacturers to get SteamOS on a number of systems other than the Steam Deck. You can buy a Lenovo Legion Go S with SteamOS today[1], and Valve will be supporting SteamOS on the Asus ROG Ally in the future[2], whether Asus partners with them or not.

[1]https://www.lenovo.com/us/en/p/handheld/legion-go-s/len106g0...

[2]https://www.theverge.com/2024/8/13/24219469/valve-steamos-as...