I suspect the answer is because computer monitors evolved from televisions and video monitors, which standardised on 4:3 and, later, 16:9 for media viewing.

There was a brief period during the switch to LED when 3:2 and then 16:10 looked like they could take over, but 16:9 made a comeback and monitors have remained mostly in lockstep with modern TVs ever since.

iPadOS also has an extensive set of accessibility options and exclusive apps like Staybl, which is specifically targeted at people with hand tremors.