Even if you know nothing about how movies get made, you know that there are very specialized tasks -- the sound guy is an expert in microphones and audio but probably couldn't be trusted to do stunts. And, you assume that when it comes to thinking up the ideas for what happens in the movie, somewhere it's all just some writer hunched over a keyboard -- a professional who is an expert in story, plot and character.
In almost all cases, the initial ideas for movie plots don't come from screenwriters at all, but from producers (basically, the people in charge of the money side of the project). So most of the movies playing in your nearest theater didn't come from some writer thinking up a story he wanted to tell -- they came from some producer saying, "There hasn't been a ThunderCats movie yet, has there?"
At that point, the producer and whoever else is involved (other producers, maybe a famous actor if they're lucky) will then hammer out a rough idea for the movie that will appeal to at least two of the four market demographics (young males, young females, older males, older females). So if it's an action movie aimed completely at young males, you throw a romance in there for the ladies. It's only then that they will give a screenwriter a call. In other words, in most Hollywood films, the writer is basically there to fill in the dialogue holes and think of clever catchphrases for Ryan Reynolds to say every time he socks a guy in the jaw.
For Example ...
The Halloween franchise wasn't cooked up by a plucky man named John Carpenter who had a dream about a man in a creepy mask. Instead, two producers approached him after they decided it would be cool to have a movie about a psycho stalking babysitters.