I'd imagine there are a lot more things that need to be done to complete a modern movie. In the 1920's you didn't have to deal with lawyers to fight for rights to use different intellectual properties. Deal with executives who want to change the view of a director. Deal with expensive and tough to work with actors. Not to mention the amount of editing and refining that are done in movies now. It would also help that movies from the 1920's where silent, and didn't often last for longer then an hour. High budget films that are 2-3 hours with 500+ people working on it take time.