Steamdeck is a gaming console not a general purpose device from the perspective of most people (yes I know it's a standard PC and completely open but that's not something most people care about). Linux is more or less just an implementation detail. Obviously not as extreme but a bit like PS4 being BSD but I don't think it's that different from Android/ChromeOS. Arch is there just to run a completely proprietary layer which is the only thing most people who use it interact with >90% of the time.
Additionally, to note that while PlayStation, Switch, Android/NDK, have some form of POSIX support, in some cases with overlapping 3D APIs with GNU/Linux, most of the studios don't bother with GNU/Linux, and rather let Valve do the work with Proton on supporting the execution of their Windows versions.
