Consider this, you have a director who makes your first movie. The first movie makes hundreds of millions of dollars at the box office, and is a huge success. Now, the director wants triple the salary to make the 2nd movie. Do you pay it? Or do you hire someone else at 1/3 the cost, and just tell him to try to make the tone consistent?

Also, directors are artists. Many artists don't want to spend 10 years making the same product over and over again. You do one movie, you get the experience, and then you move on to something new.