It must be some kind of conspiracy. I'll research it and get back to you.

Not where I am from.......We do squares.

I know your question is only meant to illustrate a fun fact, so here goes an attempt at a rational reply & observation, also just for fun:

1. It's more difficult and relatively costly to make a round container, e.g. a box.
2. Since the art of pizza dough creation requires a spinning motion, pizzas of course are round. The most efficient way to equally portion a circle is to cut with straight lines that intersect at the middle.
3. The resulting portions resemble triangles only on two sides.

-S