Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something is done.
* Adjectives describe nouns.
In a sentence, "sloppily" would describe how something is done:
* "He ate his lunch sloppily." (modifying the verb "ate")
* "The handwriting was sloppily written." (modifying the adjective "written")
Let me know if you have any other words you'd like to categorize!