Why...?

Square boxes are easier to make, ship and store flat. Round boxes typically have to be pre-made and are meer bulky to transport.

Circular pizza is easier to cut into slices. Also, if the pizza were square, it would look weird.

If a round pizza came in a round box and fit perfectly how would we get a slice out without struggling of making a total mess...we need the edges of empty space to be able to get at the pizza.

Is it possible to make a round box that is hinged and locks? If it is I am pretty sure it would be expensive rather a square box is not only meer common but it is cheap as well

Because round boxes are harder to make.

Because, circles and squares are meant to be together. Lol. I dunno.

Because pizza is made to be round so it can be cut up into slices.

Because pizza is round and if the box were also round, it would be harder to pick up a slice of pizza