Here's why:
* Adjectives describe nouns. "Poor" describes the state of someone or something. For example, "The poor man needed help." Here, "poor" describes the man.
* Nouns are the people, places, or things. You can't say "The poor went to the store" because "poor" doesn't represent a specific person or thing.
If you want to use "poor" in a noun context, you might say:
* The poor need assistance. (Here, "the poor" refers to a group of people in need.)
* He was born into poverty. (Here, "poverty" is the noun, referring to the state of being poor.)