Because our country has a number of a systemic issues right now that are all contributing to this situation. Let's step back and take a broader view at some of them.
1. College is expensive. Outside of the trades and a few other exceptions, a college degree used to be the best way for young people or unskilled laborers to get into higher-paying, more stable jobs. That math doesn't work out so well anymore, and there are continuing efforts to further increase the individual cost of a college degree. The percentage of college graduates in the U.S. has risen only a few points since the late 70s: https://en.wikipedia.org/wiki/Educational_attainment_in_the_...
2. Lack of affordable health care is preventing a lot of people from opting out of the job market. If you have any kind of illness that requires regular treatment, or even if you're worried about getting bankrupted by an injury, then you're forced to look for employers offering company health insurance, even if the pay is crap. Amazon offers health insurance for their full-time employees: http://www.amazondelivers.jobs/about/benefits/. As long as there isn't a sensible, publicly-supported health care system in the US, companies will be able to exploit this to get and retain workers at a net discount.
3. Rising housing costs in many urban areas are further squeezing a lot of people.
4. Upside-down tax structures and other financial incentives (like the ones described in this article) are further moving a lot of money from the poor to the rich. These factors together are creating a de facto peasant class, and massive companies like Amazon are doing their level best to ensure that it stays that way. It is a really good deal for Amazon when they can work this system well enough to get the public to subsidize their employees' food costs (https://yro.slashdot.org/story/18/04/21/1634237/many-amazon-...). Remember when the Silicon Valley big-wigs colluded to create a non-competitive job market for tech employees (http://fortune.com/2015/09/03/koh-anti-poach-order/)? And out of that lawsuit, the tech workers got just $5k on average. That was a fantastic deal for the companies.
So if you're a low-skilled worker, that doesn't leave you a lot of options on the table. You hear Amazon advertising (via some staffing firm) for warehouse positions in your area, it sounds like they'll hire anybody, you've heard the work conditions are bad but you figure it'll be temporary, and hey, health insurance, if you're lucky enough to be considered a full-time employee by the company.
Fix health insurance, and more people would get into the trades. Fix education, and more people would get a degree. Fix housing, and people would be less desperate for crappy work. Fix regulation, and communities would get a lot wealthier.
Unfortunately, none of that appears to be happening anytime soon, because there's a long line of people willing to argue against any of those things for one ideological reason or another, because the situation hasn't gotten quite bad enough, quite fast enough, for them to see anything wrong with it.