A director can't direct from a blank page. An actor can't perform from a blank page. No one person makes a movie. I write and I direct, and those are the credits I take. The writer is a major, major collaborator, but the writer is not as important as the actors.
Nobody's as important as the actors.
The gray area is that some directors have more of a personal imprint than others. It means nothing for marketability, so really it comes down to that director just wanting to stand out as the person who did the entire thing . . But Aronofsky said he still would tilt the scale in favor of the director. I've only done films that I've written, but when I split it up in my own head, definitely directing is more time-consuming than writing. A director lives with a movie for three years every single day, and a writer in general is about six months of work, and some writers are even under that.