Lots of reasons (and I say this as a writer). The art house reason is that film is a visual enterprise, so the visual elements are more important than the words.
Also, it’s a mistake to assume that the credits actually credit the people who wrote the move. Historically, directors were always involved in the screenplay – telling the writer what to do, changing it to fit the screen, and even writing dialog and scenes. But in Hollywood, there was a false modesty (and guild rules) that meant the director did not take credit for that work or he would look egotistical (in Europe, it was not an issue, and the director is very often credited with the screenplay).
I heard an author talking about selling his book to Hollywood. The movie went into development hell, but they hired a director and the first thing the director did was change the script. That director was fired and the first thing the new director did was rewrite the first director’s script. So directors are very involved in the screenplay writing process.
Great directors – say, Alfred Hitchcock – were involved in the screenplay from the beginning. When Raymond Chandler was working on the screenplay for Strangers on a Train, he complained that Hitchcock was always wanting changes to the screenplay and didn’t let him write what he wanted.
Chandler was also very scornful, thinking that there was no way to open the film without people thinking Guy and Bruno were strangers; he couldn’t figure a way to write it so the audience would believe it. So how did Hitchcock deal with the issue? Visually. We see the two men’s shoes (Bruno’s are flashy, Guy’s are more conservative) as the walk in the station and on to the train. Bruno is always shown first and he finally sits down. Guy appears a little while afterwards and their shoes bump together, starting the plot. Chandler thought it impossible; Hitchcock made it look easy.
Finally, the director guides the finished film. The writer might write the lines, but the director determines which line reading by the actors is more effective and what camera shots do the best job of guiding the story. The film editor puts the film together, but the director looks at it and says if it’s good or bad. Ultimately, the director takes all the elements and makes the final decision as to what is working.
This doesn’t mean there aren’t films where the writer deserves more credit (Casablanca is one exception to the idea), and film is collaborative. But the director is in charge – and great directors take charge.
Back in the 60s, Andrew Sarris came out with the auteur theory. Most people don’t actually know what he said, which wasn’t that the director is king, but that the best directors put their own stamp on their films, choosing certain subjects and themes, and this is apparent when studying their careers. (And people gave the director this sort of credit long before Sarris – James Agee’s criticism, for instance).
Sarris wrote The American Cinema, rating directors up to that time. The great directors all had a string of great movies (in some cases, a small string) and often took up similar themes.
About twenty years later, someone wrote an answer to The American Cinema, tracking the careers of screenwriters the same way Sarris tracked that of directors as a way to show how important writers really were. And you know what? The screenwriters’ best movies were when they worked with Sarris’s top directors. When they worked with poor directors, the results were poor movies (look at screenwriter Jules Furthman in this context).
So, ultimately, the buck stops with the director, and his ability to manage the film is what makes it a success or failure.
