It had to have come from somewhere. So if judges didn't make up, who did? A legal fairy. A dog. A pixie. Druids. Preachers. Who?
"As we have seen the basis of Common Law was custom. The itinerant justices set out by William the Conqueror examined the different local practises of dealing with disputes and crime, filtered our the less practical and reasonable ones, and ended up with a set of laws which were to be applied uniformly throughout the country."
You must enjoy being thrashed.