Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* "Also" and "too" indicate addition or emphasis.
For example:
* "I like pizza, and I also like pasta." ("Also" modifies the verb "like".)
* "The movie was really good, too." ("Too" modifies the adjective "good".)