Multiple writers happen like this: +1 writter: Kevin is told he can write a movie, Kevin loves this movie and pours his heart and soul into it. +1 writter: The studio likes Kevin's script, but has some issues with it, instead of having Kevin rewrite it the send it to the producer's pet writer. That producer is fired, the script goes back to Kevin with the rewrites half done. Kevin attempts to keep the material the new producer wants while throwing out all the horrible things the other writer had done -1 writter +3 writters: A director is hired for the project, he has problems with the script. Kevin is fired. The director brings in his own (2-3) script writers. +1 writter: A new producer wants to bring a hot actress onto the movie, his pet writer writes her into the script, with the movie 2 days from the first shoot. +1 writter: Nicolas Cage is in the movie, the lines in the movie are not "Nick Cage" enough. Nick Cage's writer writes more "Cagey" dialogue into the script. Every other major actor's writer also does this. Funding falls through and the entire movie is scrapped. One movie, 6 writers.
First of all that sounds like a terrible experience. Secondly, movies work very differently from novels. Movies and TV are all about collaboration, its just impossible for one person to put everything together. Books are more about an individual getting across a story from beginning to end.
I recently read a book by a longtime screenplay writer, and does describe some experiences like this.
That is actually a real story, about Kevin Smith writing the screenplay to Superman Reborn. He worked on it for a year before Paramount signed Tim Burton to direct. Burton brought in his own screenwriter, and Smith was politely shown the door. All the stuff about Nick Cage having his own writer is completely true as well. Why do you think that Will Smith and Bruce Willis always sound like Will Smith and Bruce Willis, no mater who wrote the screenplay?
Aiding an author in this way is kinda my job, so I'd expect to be paid. I don't know why anyone would devote so much time to another's project without this incentive, unless they're very passionate about the same ideas. Most of the writers here have their own projects to focus on. Also, a novel takes as long as it takes.