Well if the Republic was in full swing when Merlin furrowed his brow, it's society might have theorized as much. I can see why you don't need more than Anakin's apparently fatherless conception, but why can't there be more than that? Lucas is trying to make a myth for our time, because those old myths didn't seem to do it these days. To make it a myth for our time, it has to acknowledge the science we are currently surrounded with. The reason those older myths don't explain anything scientifically is because they thrived primarily on superstition.