And a lot of people don't understand the industry. Musicians get screwed over often. One of the saddest instances is watching Unsung and George Clinton and various members of his bands talk about how much money they never got because there was no paperwork. Because there was no paperwork, nobody really knew who did what. They were just doing what they loved.
In Hip Hop, you have session players. Talented musicians who are paid up front. If they don't understand the industry, they don't understand that the up front fee is all they get even if the song or album becomes a smash. Unless, they fight for co production credits or have someone with legal knowledge on hand, they forfeit publishing points which could mean much more money than what they received up front.
Some artists don't get that aspect as well. So they will pay session players and credit them for various instruments, but won't share production and writing credits simply because they see them as session players, instrumentalists, and musicians and not as co creators.