Here's why:
* Proper nouns are specific names of people, places, or things (e.g., John, Paris, the Eiffel Tower).
* Common nouns are general names for people, places, or things (e.g., man, city, monument).
"Football player" is a general term that describes a person who plays football. It's not a specific name, so it's a common noun.
However, if you are referring to a specific football player with a name, that name would then be a proper noun. For example:
* Tom Brady is a proper noun because it's the name of a specific person.
* Lionel Messi is a proper noun because it's the name of a specific person.