[1] Case in point: glibc's compatibility guarantees are weaker than what you get on Windows. (For instance, your system's glibc cannot be older than what a game is built against, which may present problems for devs using Fedora/Arch and players on Debian/LTS Ubuntu, something I've experienced first-hand for my apps.) The X11 to Wayland migration is also still underway. (Though things are getting better, the attitudes of some Wayland maintainers are a bit concerning: "I don't [care] what you think is normal behavior for games. You get certain guarantees with wayland. Deal with it. If clients decide to do exactly what they do on windows or X11 they won't work correctly." [3] I'm not sure game developers would enjoy such reception.)
[2] https://store.steampowered.com/hwsurvey
[3] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18...
Porting games from Android/NDK into GNU/Linux is relatively a child's play.
Playstation OS is also POSIX friendly.
Finally every serious middleware engine supports GNU/Linux.
Still the amount of studios that care about GNU/Linux is almost zero.
With Valve, there are no reasons to bother at all as a studio, target Windows/DirectX, let Valve do the work, collect the money with zero additional effort.
Now with Windows based handhelds, Valve will learn what happened to netbooks.
It's a riff on only stable OS API on Linux is Win32.