Perhaps this is a case of "divine dissatisfaction". We look at the film and Wells and see its and his genius. But from his perspective, Rosebud is just a device to make the story work. As a filmmaker he would have been through this hundreds of times to the point where it becomes somewhat meaningless. We see it once or even a few times and we see greatness. The same is true with actors. They pretty much never admit to any kind of greatness. It's something that you won't see in yourself, only in others. You might say, yeah, I'm pretty good. But never great to the magnitude that others see. I keep seeing this over and over whenever I read or hear interviews with directors, writers, and actors.