Here's why:
* Pronouns replace nouns. They are words like "he," "she," "it," "they," "this," "that," etc.
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
Example:
* Pronoun: "He went to the store." ("He" replaces the noun "John")
* Adverb: "He went to the store quickly." ("Quickly" modifies the verb "went," telling us how he went)
"How" is used to ask questions about the manner or way something is done. It's never used to replace a noun.