Why are novels laid out the way they are? It's almost as though its not as simple as "bad for reading" but rather a feature of the content.

imo it highly depends on the medium. If your content is packed with distracting ads, flashing banners, other unrelated text from other stories, then its helpful to make the columns narrow to help you scan.

If you have a more focused experience around one passage of text, wider text lets you fit more on the page and make better use of paragraph breaks and subheadings