There are vast swaths of the US that are in economic decline, and they most certainly do not allow for mobility irrespective of skill level. They don't even have sufficient economic activity to support a Walmart.
I have a hard time imaging the people are willingly sacrificing theirs and their kids' futures for no reason. It's hard to up and move somewhere without adequate savings, a family or friends network. What kind of landlord is going to rent to someone with no job? Will someone in that situation be able to get a job that even allows them to save up for a home purchase in an economically burgeoning area? Or will they always be treading water?
people have always had that mobility though; there's nothing special about the current US job market. want to become the king in a feudal society? just raise an army and take over