2. They're Only Needed for Certain Projects
Usually, a summer blockbuster does not need a huge star to become financially successful. There are exceptions to this rule (
Inception, Elysium), but for the most part brand names do the heavy lifting during that time. 2009's
Star Trek made a lot of money without having any big names.
Man of Steel stars relatively unknown Henry Cavill as Superman. Christian Bale may be a star now, but he was not when he was cast as Bruce Wayne for
Batman Begins. This large chunk of the film calendar is not dependent on star power to draw people in, allowing the producers some leeway with who they cast. My point is, movie stars are primarily needed to help give the Oscar contenders mainstream appeal. Martin Scorsese's most commercially successful pictures have been the ones with DiCaprio as the star. Do you think
Up in the Air would have made over $80 million in the U.S. if it wasn't for Clooney? The big actors are essentially brand names for smaller dramas that most people otherwise would not be interested in. The problem is, there's only a handful of these types of projects that come out each year and studios - especially the major ones - are going to offer established stars the leading roles. This makes it easier to market the film to a general audience (particularly if it is not directed by a well-known filmmaker). It's more likely casual moviegoers will be more excited for "the Brad Pitt movie" than "that one movie with a neat premise but stars some dude I don't know." Since a movie star is really only a necessity for this specific type of project, it makes it difficult for a rising star to force his way in there. Actors such as Gordon-Levitt and Hardy have made names for themselves (albeit, in supporting roles) by appearing in Christopher Nolan summer films
Inception and
The Dark Knight Rises. This is an easy way to raise your profile, but it raises concerns about your actual marketability in terms of leading a film. Last year, Gordon-Levitt got his first real shot to headline a movie in
Looper and that was only a marginal box office success. Hardy didn't have much luck with
Lawless despite becoming well-known for Bane. Maybe the tide will eventually turn in their favor, but for now it's a struggle.