Movie Posters are being used in cinemas/theatres to promote/advertise the movie and are usually destroyed or trashed after use.
Movie Posters are printed as advertising material and are not meant to fall into the hands of the public. Movie posters are usually printed in numbers needed to supply the theatres showing the particular film. The movie theatres either return them to the originating distributing company or simply throw them away. Over the years posters have found their way into the hands of individuals and dealers, leading to the posters available to collectors today. However most of the posters are still being returned to the source or destroyed after use. This makes them pretty rare collectibles.
Original movie posters are highly collectable and most gain in value every year.