You're acting like you've never accidentally reduced a soup far enough that you almost burned it.

There are dozens of cases, every year!

Counter argument: corners make it easier to pour accurately. If it's a thick soup, you're going to get strips of soup stuck to the container between the spoon strokes either way and a corner can act as a soup staging area that makes it easier to get the soup out instead of just pushing it around. And a spatula would make quick work out of either shape.

Though note that I'm picturing a rectangular container with rounded edges.

And very difficult to scrub the grease off of

I love how true this is. I made stock and froze it in a Ziploc bag using my bread tins as a brick form. Works perfectly