How would you like to work 8-hours days under constant scrutiny and pressure to pack as many boxes as you can, without official employee status (you're most likely a temp), having to pee in bottles because taking a bathroom break could get you flagged or even fired, and having to run a huge distance and scarf down your food within 30 minutes to prevent Amazon from firing you? At the end of the day I can imagine your back and arms are going to be sore from the incessant and demanding labor.
This is the definition of backbreaking. If Amazon is not going to make things better (and they definitely don't care to, they like their pennies), only organized pressure from you and your buddies through a union can. You literally have nowhere else to turn to.