Why does a round pizza come in a square box?

Why does a round pizza come in a square box?

So the corners can be empty to add anything else the customer might order! I know that for a fact. At one time in my life I worked in a pizza parlor and the corners
were' used for extra add-ons.

because making a round cardboard container (can't say box... a box is a cube...) would be very impractical.

Making it square is easy, fast and inexpensive.