It’s the Unix convention (shell expansion) for a the path to a user’s home directory:
https://www.gnu.org/software/libc/manual/html_node/Tilde-Exp...As a result, many Web servers exposed user-provided Web content using the same path: https://httpd.apache.org/docs/trunk/howto/public_html.html
Universities have continued using that convention, probably because professors have accounts (and thus URLs) for decades.
