Sometimes, while clicking around Windows, you'll notice that your system leaves behind a "focus rectangle"—a dotted outline of whatever you had last selected—that is, of course, minor, but still kind ...

The Snipping Tool is a program that is part of Windows Vista, Windows 7, and Window 8. Snipping Tool allows you to take selections of your windows or desktop and save them as snips, or screen shots, ...

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.