Because everyone keeps letting it happen. It's not illegal (yet) and the actors go for it. Acting is like gambling... Lots of people dream big, but only a few actually make it. Most of the dreamers are willing to risk everything for that chance. They'll sign any contract that gives them a chance at making it, and they think not signing a bad contract will mean they'll never work. (And that's probably true.)
The music industry works the same way, in regards to the actual content producers.
The music industry works the same way, in regards to the actual content producers.