Some web masters are convinced that they do. As a result, they insert obscene margins in their HTML; Sometimes half of the screen width goes to waste.
Example screenshots: A BBC article and A Github page.
Though they can never agree on the specific optimal number of characters per line, it is typically less than one hundred.
I could never verify that claim. Even articles that defend and parrot that idea seem to lack actual source studies, for example, 1 and 2.
Indeed, the 2011 question Are shorter lines easier to read? on Skeptics Stack Exchange could not get a single affirmative answer; the only answer states that research on the subject are inconclusive.
Apparently, one more case of "a lie repeated many times becomes true".
However, let us assume the unbased claim were actually valid. The next important question that is never, ever raised is: Would it matter? How relevant would that "optimization" be? I.e.,
Why would you want to hit PageDown (or rotate the mouse wheel, or slide your greasy finger on your touch screen etc.) more than you should need to?
Suppose one wants to partition his screen, such that he can view two (or more) windows simultaneously. I.e., he wants to put his large screen space to good use, instead of simply wasting a large part of it.
This is how stupid it looks like if margins are enforced:
"But why would you use a horizontal split instead of a vertical split?" I can see at least two possible reasons. Not all monitors are 16:9. Some windows are better viewed with large width. Anyway, it is up to the user to decide, and speaking for myself, I do occasionally use the horizontal split.