Pizza = Burger (?)

According to students in my conversational English class, pizzas and burgers are interchangeable.

The topic surfaced in our discussion about food. I asked them why pickles always come when you order pizza here. They were completely shocked by the question and remained so as I explained that this does not happen in the States. Their response:  But it's the same as a hamburger! which, in turn, completely shocked me.

They said that when talking with friends about what Western food to eat, they'll ask:  Pizza or burgers? And then they'll all agree that it doesn't really matter since they're practically the same:  bread, tomato sauce, cheese, meat, and pickles.

So, after tonight's class, I'm starting to understand the thinking behind pickles with pizza, and I'm closer to answering other questions such as why are pizzas frequently topped with mayo and corn?