The ironic part here is that NixOS and Steam don't really play that well together: Nothing is where pre-built binaries expect it to be on NixOS, including ld.so, so pre-built binaries simply won't run without patching. NixOS can do that for Steam... but as Steam is also downloading binaries, essentially being its own package manager, those binaries then run into the same issue.

OTOH you can just run the whole shebang in a chroot, exposing exactly what Steam expects (couple of libraries and the graphics drivers) which is what NixOS does and I never had any issues.

Another hurdle would be that NixOS is not end-user friendly. It just hasn't been a focus, Valve would need to write a graphical configuration/package management utility. NixOS also doesn't tend to go easy on disk space and network bandwidth which might be considerations, OTOH probably not an issue if they manage their own release channel. Things like flatpack also aren't an issue they get the same treatment Steam does.