At the risk of rehashing other comments here, software devs at Amazon are profit centers - when they work the company gains exponentially more value than it pays them. A good engineer retained can often deliver 2X to 10X the value of a new hire. The company treats them them accordingly.

Warehouse employees are cost centers. They're replaceable pseudo-robots that are filling in the gaps until the engineers mentioned above finish building the real ones. A new worker at a cheaper rate is objectively better than retaining one, because the number of boxes packed tends to increase inversely proportional with age and tenure. The company treats them accordingly.