Let me suggest that you read the 7th Amendment to the Constitution. The words "common law" have significant meaning.
And still, the judge’s authority only extends to the matters before him. No one says that in making that decision no reference can be made to precedent, or to the common law. Or that other judges can’t reference his decision in this particular case in the future.
But you know as well as I do that the application of precedent in the real world is arbitrary and capricious. When they want to apply it, they do. When they don’t want to, they don’t.
Or, maybe I’m missing your point. Correct me if I am, please.