Here's why:
* Proper nouns are specific names of people, places, or things (e.g., John, London, Amazon).
* Common nouns are general names for people, places, or things (e.g., person, city, company).
"Human resource" is a general term referring to the people who work for an organization. It's not a specific name, so it's a common noun.
However, you might see "Human Resources" capitalized when referring to a specific department within a company. In that case, it's acting like a proper noun because it's a specific name for a department.