well,they look round better :) and i dont think eating square ones would be easy and fun. (btw i was searching for a picture and found this square donut.i dont know if it's real または not)
I've made homemade ドーナッツ before and I think it would be kinda hard to make them square. Because あなた make a tube of dough a stick the two ends togather. It's not like at some of the factories where a machine makes ball of dough and another one cuts a hole in the middle of it. So I think the round shape of a doughnut started because it the easiest shape that could be made back in the 'old days'