Here's why:
* Pronouns stand in for nouns. They can be singular (he, she, it) or plural (they, them, we).
* Adjectives describe nouns or pronouns. They tell us more about the noun or pronoun.
Example:
* They are going to the park. ("They" is the pronoun, referring to a group of people).
* The blue car is fast. ("Blue" is the adjective, describing the car).