Here's why:
* Quantifiers indicate quantity or amount. They tell us how much or how many of something there is.
* Adjectives describe qualities or characteristics of nouns.
Examples:
* Quantifier: "There is much work to do." (tells us how much work there is)
* Adjective: "He is a good student." (describes the student's quality)
While "much" can sometimes appear before a noun (e.g., "much water"), it doesn't describe the water itself. It tells us how much water there is.