Studios make billions of dollars each year. Most people don't realize that the actors that are very famous are the ones making the most money but it takes hundreds of people to make a movie.
Those people aren't getting a fair cut, and because these bosses want to take as much of the profit as they can, they want to cut costs any way they can.
That's how you end up with incidents like Alec Baldwin shooting a coworker by accident or actors getting injured on the job. The more famous actors have to speak up for those who aren't that big yet.