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.

x86 makes sense, and will into the near future.