Here's why:
* It names a specific time: Midnight refers to the point in time when it's 12:00 AM.
* It can be the subject of a sentence: "Midnight arrived, and the city grew quiet."
* It can be modified by adjectives: "The dark, silent midnight brought a sense of peace."
While "midnight" can sometimes be used to describe something related to that time, it's not its primary function. For example, "midnight blue" is a color, but "midnight" itself is not acting as an adjective in this case.