There more to shooting a movie than the actual shoot. You might spend two months in principal photography and two years in pre and post production, marketing... Any popular actor or actress or even directors got a very busy schedule. If you want them to reserve a window for a project in their schedule that may or may not happen you got to pay. The bigger the name the more pull they get on that sort of things.
from reddit: the front page of the internet http://ift.tt/1seS35k