Because a majority of the new hires are working in fulfillment centers or delivery roles that scale easily.
A lot of the new workers probably work in warehouses where they are basically a kind of more intelligent robot. I know a guy who worked in one and you can learn that job in a few days. It's harder to maintain the required pace for long though.
I worked at a warehouse during college on the weekends (3 12-hour shifts Fri-Sun) and on day one was assigned to the conveyor line where I used a broom to free stuck packages and then a while later got "promoted" to gift wrapping where I stayed until I left when I got my degree.
The bulk of the new hires were most likely low skill, low income people working at their fulfillment centers