The reason is very simple. Text size. I mean not the browser/Word document text size(because those are adjustable) but the interface and Mac OS font size is very small.
Let me clear things up:
1) I have a 27" 2k(1440p) display connected to my Air m1. The problem at first was the fact that everything was so tiny to read and watch. I could go around in browsers and text editors/pdf viewers by enlarging the font, but still my mac os buttons, folders appearance and names were too small at 2k resolution.
2) Since 2k was too small, i turned into 1080P. Now i had another set of problems. Either the picture is really bad and not sharp at all but i have fullscreen view. Or the picture is crystal clear at 1080p but i have big black bars at the left and right side of my display.
3) This is when i realized that only 4k monitor or above can perfectly scale to 1080p view with sharp image. So if you want your fonts bigger then 4k display running in 1080p mode will be your only solution. As for me, i got used to the tiny font of 2k so sticking with it and that is normal. If money is no constraint, i would prefer running 4k Display in 1080p for work and 4k for entertainment.