When you write a novel or paint a picture, you have the opportunity to approach it and back off, tear up pages, write, rewrite, paint over, and come back to it. In film, once you start shooting, you can't restart the clock, and you keep moving forward, and you don't look back, and you don't go back. And that is, of course, antithetical to the creative process. It's really hard to generate a comfortable creative flow under that kind of pressure.