Pretty much any all in one meal cooked in dish can called a casserole unless it already has another name (a pot pie is not a casserole but it could be if they invented it today). It is a favorite for working moms who want to make the dish in advance then have the kid throw it in the oven and pretty much any magazine geared at them will have a casserole or crockpot recipe.

They are calling deep dish pizza a casserole as an insult. The east coast eats thin pizza while Chicago makes thicker crust pizza often with more stuff on top so to the jaundiced eye it is more casserole like.

Both are good just different and those of us without emotional attachments are free to enjoy both. Unlike chili with beans which only the uncouth eat.