This happens on windows, where pkg-config is in mingw path, but not in global PATH. And there are good reasons to not add mingw-path to global $PATH (hint: it will break everything else on the windows system).

I experienced this myself trying to use pkgconfig on Windows. And apparently others as well https://discourse.haskell.org/t/installing-a-library-with-c-dependencies-on-windows/8557/4