Round things, such as Pizza, cakes, plates, dishes, and so on, come in square boxes because round boxes cost far more to manufacture. It's a lot easier to make a square container than it is to make a circular one. It's much easier to clip a lid into a square box than a round one. Empty, they also ship and store flat, and can be stacked on a shelf (empty) when put together for that day's pizzas. For deliveries, it gives room for those little dips to be stored in the corner. And square boxes stack best inside the thermal bags for delivery.