Here's why:
* Common nouns refer to general categories of people, places, or things.
* Proper nouns name specific people, places, or things, and they are always capitalized.
Since "cookie" refers to a general type of baked good, it's a common noun.
Examples:
* Common Noun: The cookie was delicious.
* Proper Noun: I bought a box of Oreos (a specific brand of cookie).