In modern web pages, much of the content is delivered through AJAX and isn't in the actual HTML of the page, adjusting fonts often breaks layouts, and for people who can't figure out how to use AdBlock (or are on a mobile device), ads are big, obtrusive, hover over the page content affairs.
I think you're thinking about the OLD web, when adjusting fonts would break table-based, graphic-intensive websites. Unless you want to go even further back - to a time when gif backgrounds of the stars were common and text was displayed through Comic Sans.
