Don’t forget that the web-browser will request the stylesheet ... and that it will use a URL to do it, just as it requests anything and everything else that it receives.   So... the source file has to be somewhere (on the system that’s hosting the web-server) that corresponds to a URL that a web-client can issue.   And all pages, etc., must contain that URL.

The web-client does not know, nor does it care, that the file actually resides “at such-and-such place on such-and-such drive.”