Short lines can't be read in one glance, they are too short to be meaningful, so when you see it in one glance, you get no information from it. The worst case is when the line stops in the middle of a word, now you can't even see individual words in one glance.
Smaller widths make it easier for the "read the whole line in one glance" readers and is I think the main reason for this confusion.
Sometimes I like just glancing down a page and getting the gist of the screen. Other times I prefer to let my eyes do extra work and read denser text.