"Pizza" is just Italian for "pie".

http://www.etymonline.com/index.php?term=pizza http://dictionary.reference.com/help/faq/language/e02.html

And a hamburger isn't a "sandwich", it's a "burger".

But these distinctions aren't universal. It's easy to see why some people call pizzas pies if you look at variants like "deep pizza" (or "Chicago style pizza").

Essentially you might as well argue whether it's "soda" or "pop".

When referring to a pizza, Americans often use the word "pie".