Why is the word "how" considered an adverb, even if the answer is an adjective?

Consider this question and its related answer:

Question:

How was the pizza?

Answer:

It was delicious.

The question is asking how, which is defined in every dictionary as an adverb, but the answer is delicious, which is an adjective.

Why is how considered an adverb, even in this case?

The Oxford English Dictionary has a very long page of meanings for how (two senses are quoted below). Probably what you need is a better quality dictionary/grammar book!

The "adverb meaning"

I.1.a. Qualifying a verb: In what way or manner? By what means?

The "adjective" meaning.

I.2.a. In what condition or state? how are you?: (in quot. 1918) used ironically in sense ‘indeed!’ how do you do? (formerly how do you?): common phrases used in inquiring as to a person's health. See also how-do-ye phr. and n., how-do-you-do phr. and n. Also, how goes it? = how-do-you-do phr. and n. 1; how's (or how are) things (or, orig. Austral. and N.Z., tricks?) ; how do?: = how-do-you-do phr. and n.

Dictionaries generally label "how" as an adverb, though one gets into difficulties when one wants to show that it is an adverb. Adverbs modify several other word classes as verbs, adjectives, adverbs, even sentences. The only case where "how" is used as modifier I can think of is "How funny". When you look at articles about kind of adverbs you find adverbs of manner/place/time/frequency/degree/ intensifiers/focus/viewpoints/connectives; so in Longman English Grammar. But you don't find an adverb class where "how" might belong to.

But modifying another word is not the main use of "how". It is mainly used to form questions and it is appropriate to label this use simply as question words, even though such a word class is normally not listed among the traditional word classes.

By the way, answers to how-questions can be of several types. When you are asked "How do you go to your office?" you won't answer with an adjective.

How was the pizza? It was delicious.

The sentence "how was the pizza?" is what is called a surface, or S, structure. The semantics of the sentence would suggest that you were asking about the verb 'taste', e.g., "how does the pizza taste?", the answer to which the word 'how' seeks to elicit. There is a class of verbs like 'was' or 'taste' that take adjectives (like delicious) as complements, but still need a wh- question word (like how) when asking a question.

In standard syntax, wh- words are classified as syntactic adverbs just because they occur in the vicinity of the verb, a VP (verb phrase) constituent, to use the jargon.