Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things and are always capitalized.
For example:
* Culture (common noun) - the customs, arts, social institutions, and achievements of a particular nation, people, or group.
* American culture (common noun with proper noun modifier) - the customs, arts, social institutions, and achievements of the United States of America.
* The Culture (proper noun) - a specific cultural group or organization.
Therefore, "culture" alone refers to the general concept of culture and is a common noun.