Here's why:
* Adverbs describe verbs, adjectives, or other adverbs. They tell us *how*, *when*, *where*, *to what extent*, or *how often* something happens.
* In the sentence "I will see you soon," "soon" tells us *when* you will see the person.
While "soon" can sometimes appear to function like a noun (e.g., "I'm looking forward to the soon"), it is still grammatically an adverb.