The author, Rod Nave, is old, possibly retired, and probably self-taught in HTML 3.2.

Image maps were the modern tech in the years after Fulton County started pushing internet to residents---1995 or so. I don't remember there being any alternatives at the time; CSS was mostly supported by many browsers, but div tags weren't in widespread use.

For a non-programmer, it would be a herculean effort to learn and transition existing material to a new technology, and the benefits might not be obvious to someone focusing on teaching physics to 100 undergraduates most of the year and seeing the widespread, positive impact of the current implementation.