It's the same with the music industry. Like all the other comments have said, already established actors are more likely to get people into the cinema. Established musicians will play big venues because they know it's got a high probability of sell out shows (=$£€). Getting to know people in the music/acting fields has its benifits too, if I'm a director and I know an actor I've worked with is amazing on set and delivers, I'm more likely to hire them again (e.g. James Cameron films) Plus it's much easier than sifting through the masses for new talent.