That’s like saying PlayStation 5 and Switch are based on FreeBSD, so you should game on FreeBSD (well, not quite, but hopefully the point is clear). FreeBSD isn’t good for gaming, it’s just liberally licensed and easy to build on top of, hence why it’s used.

Valve has reasons to use an Arch base, and none of them have anything to do with any specific benefit regarding gaming. It’s easy to fork and maintain customized build files for since it makes so few assumptions (packages are as vanilla as possible in Arch, so it’s easier to maintain a patch set).

Valve likely has patches in SteamOS that haven’t made it to upstream Arch, and there’s likely a number of packages that are quite outdated vs upstream Arch, so installing upstream Arch will give you quite a different experience vs SteamOS.