The thing with fulfillment center work is, you tend to do one specific job all the time. If you stow, pick, move carts around, move stuff from a cart onto a conveyor belt, you do that all day, every day, until you're told to do something else.
Even though many jobs might not be "grueling," at least in warehouses where the inventory tends to be lightweight, the repetition and requirement to meet rate will ensure that even relatively simple tasks can wear people out. Having to stand in one place for an entire shift, in the Kiva-enabled warehouses, can be more physically demanding on your lower back, knees and feet than the jobs that let you move around.