Because movies have only been made for about 100 years--a history much shorter than novels or plays--in the early days films mostly relied on literary works for their stories, and a screenplay (as opposed to the director's vision) was regarded as the soul of a film.
電影百年歷史，相較於戲劇、小說，短了許多，早期電影劇本因此相當仰賴文學作品，當時劇作家才是電影靈魂。