I can see your point, but alot of the movies I've seen where the director, writter and producer are same person have usually been terrible. I think it's due to narcissism where they think they need to have control over all aspects of the film.
Although I do agree when it comes to movie adaptation of books that the story should follow the same plot. I think alot of the time movie makers exploit using the names of successful books to market their films. That often