I use to think this too but then I saw a movie that Christopher Nolan wrote but didnt direct and I did not like it (whereas I pretty much love every film where he writes and directs)
The writers can make as good a story as they like but it's still just a story. To make a good movie you need a good director. Also sometimes a bad directors will decide to discard parts with good writing or leave in parts with bad writing.
Take the Hobbit. Great story, horrible movie.