i knew two movie writers when i lived in LA. both were screen play/movie writers and told me they basically take an existing idea and make it better... its the only way someone will fork out a few mil for a movie.
sucks, but think about it though. starting a movie is all a HUGE gamble. If you saw that there was 5 other movies that did well in the last 5 years with the same plot and same story line, its a much safer ROI than something completely new. especially with costs of movies these days they dont want a big risk.
in the 40s, 50s and up to the 70s movies were a lot less to produce. I read that "Clerks" cost $10k to make in 1994, while "Clerks II" was over $5 million to produce (
http://en.wikipedia.org/wiki/Clerks_II)