What conclusion you or the OP choose to draw from that is a different question.
The jobs are designed to be doable with 5 minutes of training by anyone off the street. But they offer 40-hour weeks at above-average wages for untrained labour, with set expectations that can be met. Many sites offer after-work classes for in-demand skills to help people get better jobs. They have medical staff on-site in case of injuries, water stations everywhere to help keep people hydrated, safety rules and regulations they follow very carefully.
Most warehouse jobs do not offer those things.
>there's a line of people around the block waiting to take your job the moment you slip up.
Recruiting is typically handled by outside firms, at most sites I saw. They don't want to fire people, because hiring people costs money. Even if initial training takes 5 minutes, it takes a week or more to get good at the job, which means every time they fire someone, they lose money on labour efficiency while a new person gets up to speed.
I have some criticisms of the company after my time working there- including some that, as you said, gave me trouble sleeping at night. None are related to the way warehouse associates are treated.
Keep telling yourself that if it helps you sleep. I mean, sure, they're not the most glamorous. But what sets Amazon apart is how, as with their other operations, they've "streamlined" things, being "data-driven." By all accounts I've read, in an Amazon warehouse you're truly a number, with metrics gathered and ruthlessly analyzed. They've streamlined the recruiting/hiring process too, so there's a line of people around the block waiting to take your job the moment you slip up.