Here's why:
* Adjectives describe nouns. "Prepared" describes a state of readiness or being ready. For example: "The chef was prepared for the busy dinner rush."
* Nouns are people, places, things, or ideas. We don't typically use "prepared" to refer to a specific person, place, thing, or idea.
If you want to use "prepared" as a noun, you could use the noun form: preparation. For example: "The preparation for the party took hours."