Because, then it looks like you are getting more pizza then you actually do. I hate it when I open the pizza box and I find a skimpy pizza inside. Not only are the corners of the pizza box not filled with pizza, but the pizza circle is an inch or more smaller then the four sides of the box as well. Dirty marketing trick. It happens with powdered drink containers that are only half full. Cereal boxes, half full, food containers that have amble portions of food pictured on the cover, then there's a lot of air inside. chocolate that is fluffed with air. The more air there is, the bigger the candy bar looks and the more they can charge you. Pretty soon they'll be charging you just to smell the food, candy, etc., and only the rich will be able to afford a decent meal. Sheesh!