If there are so many writers out there, and studios bring in so many people for rewrites, why do such a large percentage of films end up with mediocre to terrible scripts?
Not really, Diunx.
That's a loaded question, and one that someone could write an entire book answering. And it's really the wrong question - it should read, "Why do we end up with such a large percentage of awful films?"
And that's because making a good film is a total crapshoot. You need a good writer, a great producer, a strong director, a talented DP, clever editor, collaborative crew and a solid cast. Getting all of those elements lined up, and staying within the framework set by a studio, is pretty fucking difficult.
An awful cast can make a great script look bad, but so can a crew that is temperamental. If the whole project is held up by a producer that has no confidence in his crew, that person can also undermine the entire production.
That's not including insidious things, like studio notes and screenings. Hollywood is nothing but a bunch of stories about films that are ruined by notes and stupid executive decisions. My favorite example of this is William Peter Blatty's Exorcist III: Legion. Go look it up, it's a fascinating production. And I think a book is coming out this year.
... but filmmaking is a very difficult thing. Everything has to line up in order, and it's rarely just the screenplay that is the undoing of any one film. Great, imaginative directors have propped up substandard scripts numerous times. And great screenplays have also propped up mediocre productions.
I'll give you an instance of something that happened to me today. I'm working on a project, and one of the producers (who is very suit-ish), read the latest draft of one my scripts. He tells me he loves it (with an exclamation point, no less),
but...... and proceeds to write a bullet point list of a dozen fucking pointless revisions. Really stupid stuff. Like, I had a character with no dialogue at the end, because it was unnecessary, but he wants to include a scene where he looks at the camera and says this truly awful line of dialogue (that he wrote). And this is a very small production, so imagine what it's like working for a major.
And Methodis is correct that there is a large reservoir of uncredited writers that work on stuff, usually to address studio concerns and not to really lend their strength to the screenplay. Most of the credited guys turn out meddling stuff at best because they know it will sell.
Truly imaginative stuff is a tough sell.