Line length/text density is mostly limited by visual acuity in arc seconds or arc minutes, not by pixels on a display. There is, of course, doubtless a wide range of human visual acuity, but most people wear corrective lenses to target a much narrower range. Hence you hear "120 vs 80" and never "800 vs 40". Font choices influence this, too.
It's just a "complicated unit" to translate into how things are selected and even varies by "sitting distance" from displays. So, people don't use it, but it's absolutely what drives all this. Book/Newspaper/magazines have had very high DPI (really dots per arc degree!) for many more decades or even centuries. They limit to what is "easy to read" for their market, but their problem is a little different than "logically stiff" source code. Computer displays (combined with font designs) basically got to 80 columns by the mid to late 1980s and stayed there even as pixel densities have ever increased since then. People do wider aspect ratio displays now and so a pair of side by side 80 column windows is not so rare. Had "book page" aspect ratio like "portait mode" displays been more common earlier on, things might have settled on 64 character wide windows. So, maybe be happy it's at least 80. ;-)
