Here is a definition of the word "agency":
action or intervention producing a particular effect. Google attributes this to Oxford Languages.
Merriam-Webster gives
the capacity, condition, or state of acting or of exerting power.
You seem to be asking "Why do we need the word
agency to refer to the phenomenon of agency?" Which strike me as an odd question.
Naturally enough, substituting rough synonyms will roughly preserve meaning.
The threat topic is
What is player agency to you? To me, it is the capacity to participate in establishing the shared fiction. I have been clear about that since at least post 215 (see my reply to
@clearstream just upthread).
Here is my post 219. which sits two posts below your post 217:
I don't understand why you object to me using an ordinary word of English with its ordinary meaning.