

Humans (and most other animals) see better side-to-side than up-down. Your eyes are spaced horizontally, giving us a wider horizontal field of vision. People generally prefer putting things side-to-side in work environments, maybe also reflecting how much easier it is to move and work within a horizontal plane than a vertical one. So the upper threshold for monitor width would be longer than the upper threshold for monitor height.
That being said, I know reading is best done in narrower columns, to reduce the amount of left-right movement your eyes need to do which can cause you to lose your place when skimming lines. Three columns of text on a 16:9 monitor is way more readable than one column of text that spans the entire monitor.
And then why do we make an exception for phones which are predominantly used in portrait mode? I guess maybe just for easier 1-handed use? Maybe also to give us more peripheral vision of potential hazards and other things happening in the background when using them, since they’re mobile devices.
“If you had just picked a better water bucket, we wouldn’t be in this mess. It’s your fault if the house burns down.”