Those artist need money to pay you. You don't work for free, do you?
It is a capitalistic world. You know how it works.
If I was an established artist making a good living from hip hop I would PROBABLY agree with you (depending on how much potential I saw in an artist).
But thats not the case for me or Dark Knight...
Unless Dark Knight is a successful producer that has a line up of people waiting to pay him big money for his beats. In which case he could easily turn down opportunities.