I am in no way saying that shooting a film is easy....having worked on quite a few of them, even some of these "big" ones you mention I know firsthand how difficult and challenging it can be. I am simply saying that there is a difference between shooting something fresh and then having all the elements (green screen fottage of actors and background plates and CGI effects all as separate files) and trying to work with and change an already finished product. In one respect one is easier to do than the other....I am not saying films are easy to make.