Given what appears a nearly willful historical amnesia, it is easy for the popular imagination to suppose that existing political and social conventions have always been so, or that preceding social norms are by necessity inferior to the current "enlightened" perspective promulgated via public education and popular media.
As such it would be inconceivable for many that "the law" has any other source than legislative bodies. For an American the answer to the question "Where do laws come from?" is, if goes beyond a nebulous "the government," is Congress, where democratically elected representatives compose the rules of society. Before that, if before that is even considered, kings ruled arbitrarily with absolute power, always fighting wars and throwing people in dungeons.
But the idea that law is either the will of the king or the work of the legislature is a very recent idea, arising in America at least, in the early 20th century with the Wilsonian concept of democracy as a transcendental ideal and the ultimate "good" form of government..
Today, while reading, On the Impossibility of Limited Government and the Prospects for a Second American Revolution by Hans-Hermann Hoppe, I came across a reference to Kingship and Law in the Middle Ages By F. Kern. Translated by S. B. Chrimes. 2nd imps., 1949.
I did a little digging and found this nugget.