Here's why:
* Adjectives describe nouns. They tell us more about the noun they modify. For example, "the big dog" or "a red car."
* Adverbs describe verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done. For example, "He ran quickly," "It was extremely hot," or "She spoke very slowly."
However, there are exceptions:
* Some adverbs can function as adjectives. For example, "the early bird gets the worm" ("early" describes the bird).
* Phrases can modify adverbs. This can sound a bit awkward but is grammatically correct. For example, "He walked with surprising speed" (the phrase "with surprising speed" modifies the adverb "quickly" implied in "walked").
It's important to remember that the core function of adjectives is to describe nouns, while adverbs describe verbs, adjectives, or other adverbs.