Most pizza is white bread, marinara sauce, cheese, and less healthy toppings like red or cured meat. Basically pizza is carbs and fat, minimal micro nutrients, and moderate protein. The amount most people need to eat to feel sated is generally more calories than a meal centered around beans, lean meats, or other proteins.

There is also a lot of variability. Deep dish pizzas or some pan pizzas from some chains are soaked in oil in the pan when cooked (looking at you pizza hut). Certain sauces also boost the calorie count, such as ranch or Alfredo.

Likewise, a thin crust pizza with lots of veggies is comparatively ok in terms of calories per slice.

I always mentally treat pizza the same a pasta. It's a potential to carb and calorie bomb yourself without feeling sated for long, but can be prepared in a way to be better and part of a rounded meal. Eating a couple slices with a salad is fine. Eating a whole pizza on to of other regular meals in the day isn't.