Here's why:
* Adjectives describe nouns. For example, "The large house" - "large" describes the noun "house".
* Verbs show actions or states of being. For example, "The dog barked loudly" - "barked" is the verb showing the action of barking.
While "large" can be used in a phrase like "to large it up" which might sound like a verb, it's actually a slangy way of saying "to increase" or "to make something bigger".