You're assuming that people can relocate on a whim, considering the rental prices right now? If an Amazon warehouse is what pays the most at a 30 miles distance, that's where people are going to go.
The "job shortage" isn't distributed equally on the surface of USA either.
It also takes a lot of resources (time + money) to get the skills necessary to enter a profession or some skilled labor job.
tl; dr: The alternatives are often another similarly poor job, losing whatever paltry support the government is providing, and/or being one medical condition away from financial ruin.
The same happens in higher qualified jobs too. Why do so many programmers work in abusive companies while there are better companies out there? Maybe they lack the initiative, qualifications or just understanding to apply at better companies. Whatever it is, the fact that people go for those jobs shouldn’t be an excuse for abuse by employers.
Otherwise we could tell abused spouses the abuse is ok because they should have left if they don’t like it.
Food service in particular, I think, is having a lot more difficulty hiring and retaining people than they're used to, even with considerably higher wages and hiring bonuses, but if you believe what you read in the news then it sounds like Amazon warehouses aren't (otherwise you'd think they'd ease up a little on being hardasses). These aren't exactly the same type of job, but they're "pretty similar" in many ways. So, what's the difference?
Location is a factor. Some of the warehouses are located in the far outer suburbs, making them the largest employer in some areas/towns.