Because the social safety net in the US basically requires one to work a job (with Senators even arguing for work requirements for the child tax credit!). Employers are in on this as well, so they know they can offer the bare minimum in terms of salary, safety, benefits, etc.

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.