Pizza is a savory dish of Italian origin, consisting of a usually round, flattened base of leavened wheat-based dough topped with tomatoes, cheese, and various things.
The answer is actually quite simple: square boxes are easier and less expensive to produce, because they be assembled from one sheet of cardboard.
Also, The square nature of the boxes makes them easier to store, since they are more stable than round boxes.
It would be very difficult and demand a lot of work to not only manufacture, but also construct the round box on site. Furthermore, it would be expensive to ship/transport these fully constructed round boxes.