And every prequel has that tagline. George Lucas used that tagline for Star Wars Episode I and literally every entertainment franchise since then has also made note of this truth. The makers of Star Trek, James Bond, the Exorcist, King Arthur (KING ARTHUR!) have all asserted this reality since, as if this nugget of wisdom was somehow controversial.
Maybe you were under the impression that legends sprung into the collective consciousness fully formed, or even that they are shaped over time by various different tellers and that definitive versions are elusive, if so you are wrong: They have beginnings. It is critical that we are clear on this point.
The one that gets me (perhaps even more than King Arthur) is the Texas Chainsaw Massacre. It's clear that series is running out of steam due to the fact that they released a remake like two years ago, followed by a prequel to that remake. Because continuity and character development are really important to that particular fictional world.
I have a new idea to reinvigorate that series, the next entry will be a Ken Burns style documentary. An old man will sit on a rocking chair (the obligatory tumbleweed will blow by) and the filmmakers will solemnly ask him for his perspective.
"Well," he will say, "Once upon a time, when the legend began, there was a chainsaw in Texas. It's funny how now, in retrospect, you realize that a massacre was almost inevitable..."