Here's why:
* Verbs describe actions or states of being. "Shouting" is an action.
* Adjectives describe nouns (people, places, things).
Here's an example:
* Verb: He was shouting at the top of his lungs.
* Adjective: The loud sound of his shouting startled everyone.
In the first sentence, "shouting" is the action. In the second sentence, "loud" is an adjective describing the noun "sound".