The problem with the question is that it assumes that scripts are rewritten because they are "bad." Here is the answer that I wrote on Quora:
Both my brother and I have had the experience of selling scripts that were completely re-written. The movie "They" was a spec script that was bought entirely as a result of the evocative and imaginative writing, yet the movie that eventually came out contains only one word from my brother's original script: the title.
The same thing happened with his second produced film, "The Deaths Of Ian Stone."
You can read his brilliant original screenplay for the movie “They” here. The spec script bears no resemblance the film that was eventually produced. Now, read his sobering interview here about the release of his follow up movie “Deaths of Ian.” The bottom line is that “they” took his screenplays, not once but twice, and rewrote every word.
This is a very, very common experience. There are a number of reasons why it happens. Here are a few...
- Scripts are not rewritten because the "writing" in the original draft is bad. Scripts are rewritten because somebody on the team: the producer, the director, the Lead Actor, the various studio executives have ideas about how to make it "better." Once hundreds of "little" changes are made, everything is new, and the original script is gone. It's like the Chinese torture "death by a thousand cuts."
- A Studio needs to make money. Very few people understand how astoundingly risky it is to fund a movie. (To get an idea of just how risky, read this article HERE) Often it is precisely those elements in a script that are shockingly original and evocative (the reason the script was bought in the first place,) which make it extremely risky at the box office. So those core elements are often the first things to go.
- The original writer is usually replaced. Because the original writer will often be resistant to a new "take" on his or her material, the studio will hire a new writer to make the changes. Check out my friend Neal Edelstein's (Mulholland Drive, The Ring) superb answer on Quora to Why do so many big budget Hollywood movies seem to be built on scripts that lack depth and character development?
- Studio executives are often replaced. Studio executives from d-girls, to VP's, to studio heads are fired and replaced as swiftly as new writers. So a script may be changed one way to suit one group, and then another way to please the next group, and then mostly likely be thrown out all together by the new guy taking charge.
- Producers, Executives and Directors consider themselves "the author" of the film. When a spec script is bought, (unlike a novel or a play), the writer gives away all copyright. So the studio now can change the script in any way they see fit. The writer (usually) has little power and little say over this process. A director in particular may consider him(her)self the auteur, and make numerous changes to reflect his/her "vision."
- It is to the advantage of subsequent writers to change as much as possible. I have experienced this from both sides, having had my original scripts rewritten, and having been hired to re-write other writers' work. In order to get credit on a movie, a writer must have changed at least 50% of an original screenplay. Otherwise, he or she may work months or years on a film and get no credit what-so-ever. The guy who brings the director his coffee gets credit, but the script doctor who wrote 40% of the script does not. Without credit, the writer is effectively anonymous - no acclaim, no boost to his career, no awards, no residuals on DVD sales, nothing. As a result, many writers try to re-invent the script - changing major characters, plot structure, and theme - in order to win credit.
- The domino effect. Even when executives and filmmakers try to stay close to the original script, some changes just have to be made to suit the budget or the practical realities of shooting the film. However, when one little change is made, another has to be made later in the story as a result. This change necessitates reworking another scene, which no longer seems as good, and so on and so on...and the original scenes drop like dominoes.
It's just the way the movie business works, and the reason more and more screenwriters are considering writing books. :)