Here's why:
* Adjectives modify nouns: They describe the qualities of a noun. For example, "blue" is an adjective because it describes the color of something.
* Determiners specify nouns: They tell us which noun we're talking about or how much/many. "All" specifies that we're referring to the entire quantity or group of something.
Examples:
* Adjective: "The blue car is fast." (Blue describes the car)
* Determiner: "All cars have wheels." (All specifies that we're talking about every car)
While "all" can sometimes seem like it's describing something, it's actually quantifying it. It tells us *how much* or *how many* of the noun we're talking about.