Maybe because the myth of Lucas’ master plan persists, when he too was making it up as he went along?
Ah, but he didn’t make everything up. His master plan for the first trilogy got reworked a lot and things got added along the way, but the end game was a huge planetary battle and a space battle at the Death Star. He stole the Death Star from his original and moved it up and then reworked the middle and expanded the ending ending up in the same place and adding the redemption of the father to his mythic tale.
The plan for the PT was obvious. What he planned for the ST is a mystery. We know a self-exiled Luke and training a new apprentice, but not much else. But while Lucas made up the individual stories as he went, he always had a larger plan. Making Vader Luke’s father made him create the basics that led to the PT which he pretty much followed, though as we all know he made some interesting changes along the way. I’m hoping they kept that larger arc from Lucas’s treatment. What Kathleen Kennedy said is they started with that (they didn’t just throw it out) and the worked from it to expand and develop the story like they do with every movie (not all movies start as a completed script) and they moved beyond a lot of Lucas’s ideas. So while Rian wasn’t working to much of a plan, there was a plan. As he said, training was for the middle. That is what we got. I just don’t really trust Abrams to formulate a satisfying ending so I’m hoping whatever endgame Lucas had in his treatment is the basis for what we get at the end of IX. If they mix Lucas’s mythic end game with Abrams story telling, I think they might have a good final chapter. If Abrams is creating the ending out of his head, I don’t think it will be what fans hope.