While it may be insanely lucrative, Hollywood films are also very fragile. It doesn't take much to derail a production, be it actor drama, budgetary concerns, or any number of other issue. And even when a film is completed, a single scathing review or rumor of on-set drama can drive audiences away in hoards. On top of that, hundreds of millions of dollars are spent to make these movies.
There are some surefire hits that are safe for studios to fully invest in and horror films almost always make a profit because they can be made cheaply, but just about every other film has virtually no certain guarantee of even making its money back.
And nobody knows just how vulnerable the movie industry is than directors. They have to oversee every minuscule detail of a film throughout all stages of its production and pour a not insignificant amount of passion and effort into their work. So when it comes to defending their reputations, promoting their movies, or, above all, sabotaging their industry peers and competition, nobody can outdo an experienced director.