Because the pizza cutter wheel is inferior to a knife.
@BoomerClone pay attention.
If you start the pizza cutter outside the pizza, it's too difficult to go over the crust and at the inverse side you have a tendency to pull up near the end up your "slice". So they tend to look like the top right and end and/or start early. Thus not fully cutting the pizza.
That's why the downward force of a knife is way more useful. You will never need to go back and forth until the pizza is cut like you do with the wheel.
View attachment 128938