Here's why:
* Nouns name people, places, things, or ideas.
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often describe location, direction, time, or manner.
For example:
* The book is inside the box. ("Inside" shows the location of the book in relation to the box.)
To use "inside" as a noun, you would need to make it into a noun phrase by adding a noun after it.
For example:
* The inside of the box is empty. (Here, "inside" is part of a noun phrase that refers to the inner space of the box.)