For a computer, even if you could fit 4-8 text columns on a 5K monitor... What do you do if the content demands a 5th-9th column? You'll have to scroll.
As someone else pointed out in the first iteration of my design here, using non-standard behavior for the margin notes in the mobile view meant it took experimenting to figure out that tapping on a cross-in-a-circle icon would expand additional flavor text. A distinctly un-cool surprise.
When presented with a multi-column text layout, would a user know how to intuitively use the site? They're trained to look for a scrollbar on the right side of the screen to indicate "Hey, friend! More content below." Would you keep adding columns and scroll horizontally indefinitely? There are a lot of challenges to overcome in that design.
The appeal of the Tufte-styled site, for me, is the side notes/margin notes. The whole site is ~1200px wide in three columns. 200px nav, 600px text body, and 315px side notes--the rest is padding between columns. The design takes advantage of the "wasted" space to add in delicious flavor text that I'd otherwise cram in with loads of em dashes and lengthy parenthetical asides.
I designed the page on a/for a 1080p screen, assuming most 4K+ users are used to seeing websites hang out in the middle of their massive TV-sized displays.
Someday, though, it'd be interesting to see if there's a way to combine more information density on a high resolution display without sacrificing readability/usability.