They're a plan for making a movie, but not the movie itself. While the author of a novel has the final say about everything that happens in a story, the screenwriter is by default only one of many hands in making the movie, and everyone who becomes involved with the project will change it in one way or another. Thus the screenplay has to communicate the overall vision for the movie, above and beyond all the details of character, plot and theme. In short, a book is just a book, but a screenplay has to be a story, a plan, a sales tool and a mission statement all in one. Fiction can ramble.
Screenplays have to be ruthlessly efficient.
In fiction, the author can say what a character is thinking. In movies, a screenwriter doesn't have that option, without resorting to some device like a voice-over or flashback. The reader of a book can put a book down and think about it, or flip back a few pages if something was confusing. Sitting in the theater, the audience doesn't have that opportunity. Therefore, the screenwriter has to be extra attentive to make certain the audience will be able to follow the story at every moment.