Because a written script can be "adapted" many different ways visually. The script dictates dialogue (although that often gets changed on set) and story, but the director is responsible for all departments and their creations to make that script come to life and driving the overall "vision" of the project.
I think in script issues too, on a film the director is higher ranking so can usually dictate changes (I think). Whereas on, say, a TV show the director is largely bound to the script and the writer has more power.