Okay, then why aren't all these workers quitting their hated jobs in droves? Why would they care if they get fired, when they can just get another, even better job? If there were better jobs readily available to them, then Amazon would have no power over these workers.
"Why would they care if they get fired, when they can just get another, even better job?"
Have you considered... they cannot get an 'even better job'? I highly recommend reading "Hustle and Gig: Struggling and Surviving in the Sharing Economy". It details the lives of people working at TaskRabbit, Uber, AirBnB, and KitchenSurfing. It might also help explain the choices of why people are taking 'bad work' (that there isn't 'better work'.)
