I can't get on board with calling a pizza a "pie". It's not a pie. A pie has height, and it has a pastry crust on top. If it has height, but no pastry crust on top, then it's a tart. Chicago-style pizza could therefore be considered a tart, but a regular pizza needs to be more or less flat. It is cooked on a flat pan, not in a pie tin.
This is my opinion, of course - see my recent apology for seeming to proscribe language usage based on my personal opinions. A pizza is flat, with no crust on top - I make an exception for Chicago-style - and is therefore not a pie.