It's missing

   <meta name="viewport" content="width=device-width, initial-scale=1">

Not sure why we need to add that but mobile sites look like ass without it.

(Or more precisely, not sure why "make the site look like ass" was chosen as the default, and making it look good is opt-in!)

The site is using an obsolete version of XHTML. So it's probably using out of date other things, too, and not targeting phones.

EDIT: I just checked. There are 77 HTML errors and warnings alone.

Because browsers, especially phone browsers, are broken.

A lot of older websites will look like this on a phone, because they don't have the viewport meta tag, which wasn't a thing until mobile phones became sophisticated web browsers.

Firefox reader mode will probably render it OK.