There was a whole era of software UI that actually had personality.

Snipping Tool, Windows 11's default screenshot utility, will soon get another neat feature, allowing you to add shapes and forms, such as arrows, squares, lines, circles, and more to your screenshots.

Maybe they were just easier to render in older GUIs, and it has just carried on. (and you couldnt call it windows anymore) Because it's easier to describe a square than some flowing wavelike shape.