The major difference is that in the era of print, it was pretty logical where a multicolumn wide layout could go like on a newspaper, but in an desktop experience the browser markup is theoretically endless.
> actual user studies to show that wider text is harder to read
That may apply to most people, but not to everyone.
margins to control the width of large blocks of text have a ton of research in their favor, it's not just "more whitespace = more gooder" UI design madness. there's some of that of course, but there's a sane core underneath it all.