Here's why:
* Meaning: The word "knife" is a noun, and it usually functions as the main part of a phrase.
* Compound Word Formation: Compound words are typically formed by combining two nouns, an adjective and a noun, or a verb and a noun. In these combinations, the first word usually modifies the second word.
While "knife" can be part of a phrase, it rarely acts as the second part of a compound word.