That's a great question. I'll think on it a bit. Tough to see one without the other. I guess the first thing I can think of is - that a film with great direction, might still not be the best picture. I'm no expert on the subject, and I certainly don't understand the mechanics behind the awards themselves... but I can see how you could pull director out as a part, and best picture be judged on the sum of its parts.
But what part can you pull out of a film that you can say was great direction? How do you judge a director's work APART from the fact he's the creative head of every other part of the picture, from the acting to the cinematographer.
I've always wondered this, too. Sometimes I think that best pic is the Academy's choice for best overall movie, and best director is some kind of insider pat on the back. From what Wikipedia says, the whole Academy can vote for best director of the named nominees, but the nominations are made by the Directors' section of the academy.
I think you're assuming that he/she is the total and complete creative head of the picture, when in fact, many of them are not. What if a film had great direction, but a terrible soundtrack? What if a film had great direction, but terrible set design or art direction? I agree with you that it is difficult to separate the two, but I think you still can.
>What if a film had great direction, but a terrible soundtrack?
>What if a film had great direction, but terrible set design or art direction? Then the film didn't have great direction. It's impossible to seperate the two. Every director works differently, but the entire film is the director's responsibility. He's the one big picture person on the project. In the late 20s when the academy was invented, through, the producer was the creative head of the picture, and the director's award made more sense.
I'm not in the industry. I don't fully understand the academy. However, I think I understand it well enough to know that while directors may have creative control, it is not absolute. Producers and Executive Producers overrule them on stuff all the time. Things that directors want, end up on the floor. Look up Alan Smithee. As I understand it - very few directors are given complete artistic control.