It could be Arm64, but the games aren't. Not many devs or studios are compiling for both x86 and Arm, so unless you want to emulate the instructions, x86 is the preferred platform.
I'll reply to my own comment since so many people decided to give me shit and not understand what I was saying:
Valve is primarily a software distribution company, not a hardware company
they went with x86 because it makes sense since not many studios are distributing arm64 compiled versions of games
Distributing multiple versions of games in different architectures is HARD
instruction emulation is SLOW
there is no reason why Valve would go with a custom Arm hardware platform when literally 99% of all gamers are running x86
running an emulation layer to translate removes all power savings benefit for the most part, especially since in gaming it's almost all GPU instruction taking the brunt of the power draw
shipping multiple architectures on Steam is equally HARD, as you're not just debugging one architecture, you're debugging multiple, and then variants of Arm versions.