Here's why:
* It names a concept: Syntax refers to the rules governing the structure of sentences in a language.
* It can be the subject of a sentence: "Syntax is a fascinating area of linguistics."
* It can be modified by adjectives: "The complex syntax of the poem made it difficult to understand."
While we talk about "syntax" being used in a sentence, it's not the actual word that's acting as a part of speech within the sentence, but rather the concept it represents.