Some people deny this-but it’s true-NOBODY, and I mean nobody-eats pizza for snack. No normal people eat it as a snack. People have seichal and they know that they are eating a meal. A meal doesn’t mean a 4-course-dinner, even just one dish is still considered a meal. So you have to wash on even one slice.
Now, where does making a mezonos come in?
When the pizza is made with fruit juice so it’s not considered dough l’halachah. Most pizza shops don’t use fruit juice, so it’s not a common application for everyone, but for those that do eat from pizza shops that make their pizza with fruit juice, they make a mezonos.