Here's why:
* Adjectives describe nouns. They tell us more about the person, place, or thing the noun refers to.
* Nouns are words that name people, places, things, or ideas.
Example:
* The cranky old man (Here, "cranky" describes the "old man", which is the noun)
If you were to use "cranky" in a sentence as a noun, you might say something like: "His crankiness was legendary." Here, "crankiness" is the noun, referring to the state of being cranky.