The reason is shapes with square corners are structurally weaker because the corners or square windows are a place where stress can build up. Arcs and circles are considerably stronger because they are better at distributing forces equally. If port holes were square, they would get worm out rather quickly.
source https://rb.gy/a8d8ou
One of the earliest commercial plane called de Havilland Comet crashed more than once until they figured out that the square windows were the weak point.
source https://rebrand.ly/06gw0gs
