They don't. They just mention the studies. I was wondering if it's linked to legacy terminals because ...

... i don't know what kind of books you read, but all the books I read have as much text as will fit on the width of the page.

The only place i've seen thin columns was in technical books with introductory material. Never in fiction or advanced technical books.

I don't know what books you've read, but the ones I have read don't have pages which are 0.6 meters wide like my monitor. And paper newspapers, which come the closest of any medium I have used, use many columns per page to avoid ridiculous column widths. We can debate whether the optimal column width is closer to 50 characters or closer to 150 characters, but at almost 400 characters we're so far outside of what studies find comfortable that the discussion just becomes silly, IMO.