Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. "Never" tells us how often something happens (not at all), which modifies the verb or adjective it describes.
* "In" can be a preposition, an adjective, or an adverb depending on the context. But "never" always functions as an adverb.
For example:
* "I never go to the movies." "Never" modifies the verb "go."
* "It's never too late to learn." "Never" modifies the adjective "late."
* "She's never really been happy there." "Never" modifies the adverb "really."