Here's why:
* Adjectives modify nouns, providing more information about them.
* Demonstrative adjectives point out specific nouns.
For example:
* "These apples are delicious." ("These" modifies the noun "apples")
Nouns are words that name people, places, things, or ideas. While "these" can be used to refer to things, it's not the noun itself. It's the word that points out which specific things.