I don't know for sure, but I think it's installed under /nix thinking of multi-user Nix installs. With Nix any system user can install its own set of packages a

I don't know for sure, but I think it's installed under /nix thinking of multi-user Nix installs. With Nix any system user can install its own set of packages and on NixOS there are build users that install packages on behalf of normal users.