Here's why:
* Noun: It refers to the quality of being fluent, which means speaking or writing easily and smoothly.
* Example: "She has impressive fluency in Spanish."
While you can sometimes use "fluency" as an adjective, it's not its most common usage:
* Adjective (less common): You might use "fluency" to describe something that is fluent, but this is less usual than using the noun form.
* Example: "He demonstrated a fluency of speech."
There is no verb form of "fluency." You would use the verb "to be fluent" to express the same idea.