Because the nix package manager places all system packages under /nix/store/uniquehash-packagename-version/
Where the unique hash is obtained via a Merkel tree of all the inputs. So in particular, binaries and libraries exist underneath those directories, not in the places you would expect from FHS.
In order to make the system actually work, environment variables are set up and executables are patched to refer to specific paths within the Nix Store.