Here's why:
* You can have multiple casseroles: "I made three different casseroles for the potluck."
* You can count them: "There are five casseroles in the oven."
* You can use the plural form: "We had a choice of several casseroles."
However, "casserole" can sometimes be used in a more general sense as an uncountable noun, but this is less common:
* "Casserole" as a dish type: "I love casserole, it's so easy to make." (Here, it refers to the general concept of a dish, not a specific item)
In most cases, treat "casserole" as a countable noun.