In short, most of the necessary line items on Arthur’s monomyth c.v. don’t come together for at least a hundred years. And even when some of them do, others fall away, picked back up in some new combination later, hardly ever all present in the same version of the story at the same time. Look close enough at the other heroes and you’ll find the same holds true of them, too. Superman hit newstands in April of 1938, but Jor-El would have to wait until early ’39. Heck, Supes couldn’t even fly until 194121 –a trick he stole from Captain Marvel, a character first imagined by a rival company as a Superman ripoff! Actual origins are messy things, full of just this sort of double-back borrowing.
Devoted disciples of Campbell could try to pull off a double double-reverse juju to answer this, arguing that their favored list of characteristics accretes upon established heroes precisely because of some still deeper primal yearning, that the Hero With a Thousand Faces is the statue hiding in the marble block of our unconscious, waiting to be chiseled free by successive generations of artists who each wield tiny, inadequate chisels. But there is another model: making it up as you go along (while stealing whatever you can get your artistic hands on).22 It lacks the panache of monomyth, I admit, but it has the virtue of hewing a bit closer to the facts. And it certainly describes how Geoffrey of Monmouth seems to have operated.