It’s kind of for the same reason studios keep pumping out sequels and remakes. Financing films is expensive and a lot of them don’t turn that big of a profit so if the original was a hit they’ll keep making them until they stop being profitable. I think for directors a lot of it also has to do with who you know and lucky timing. In Brett Ratner’s case he was friends with Russel Simmons and when the original director of Money Talks backed out Russel Simmons suggested Ratner. He’s been making studios money fairly consistently so he keeps getting work.
It’s hard for new film makers to break in because very few people in Hollywood want to take a chance on someone who has yet to prove themselves. You’ve got to really knock people’s socks off to get your foot in the door.
