Hypotheses abound but we don't know any answers.
I don't know when the putting green became part of the game - I have not been here that long. They may have felt it was required - I don't know.
We don't know the arrangements for new (WC) or disappearing (Oakmont) courses, so we guess.
There is no sense in guessing and arguing - WGT has heard this request numerous times and they have yet to act on it. They might add one tomorrow - or never. After this period of time, I'm not betting on it happening anytime soon.