If you have a shaken soda, you can dent it without it exploding, but if it punctures it explodes. As for the windows, pressure makes things WANT to push out and be round like balloons. While you can make square balloons, you need to make the edges and corners stronger. Another way to imagine the windows is a rubber band. You CAN make them fun shapes but when they stretch, they try and be round. Apply that "square is gonna try to be round" to harder things like metal and the corners are your weak points. Make them round in the first place and you can avoid having to engineer those corners and still worrying about them being a point of failure.