Children and family/friends who are around to support you and also sometimes watch your children are often a reason. Could someone move themselves and their two children? Yes, it would cost $300 (to start). Then you'd need to find a place where all three of you could live before you've actually even found a job. This short-term place to live would likely be a motel, meaning you'd likely be unable to cook and spending more on food to feed yourself and your family. Then you need to figure out child care because you've lost the support network of family (and friends) who were living near you.
Physical mobility, moving from state to state, in this country is often much harder than the simplistic viewpoint people seem to give it.
If your perspective is that of a single individual with no dependents (even if you're poor) then you're going to think that physical mobility is relatively easy and everyone should be able to do it.
If your perspective is that of an individual with dependents then you're going to think that physical mobility could quickly become a challenge if you can't afford the upfront costs and short to mid term costs after you've moved and need to "settle" yourself.
There are a lot of, "Sure, but..." responses that usually come about with this type of discussion. "Sure, but why couldn't they temporarily go into debt? Their long-term earnings would put them in a better place eventually." and "Sure, but why aren't they applying for food and/or housing assistance or other programs at the location they're planning to move to." or "Sure, but why couldn't they have family watch their kids while they go to the new place and get setup."
The reality is those perspectives are from someone educated enough to understand economics, government programs and how the system works, and someone with a supportive well functioning family. All of which someone working a food-service job is unlikely to have.