Not everyone can drive. Many households only have one car between multiple adults. Some can't afford childcare and rely on nearby relatives to help out, and would have to start paying for childcare if they moved. Some people don't have the education or job training that would allow them to get a job worth commuting to. Some people are reliant on social services or medical care that is not available otherwise. Many of the suburbs around American cities are just as expensive as the cities themselves but without walkable areas, nearby shops, or any public transportation.

This is just a handful of examples and it barely scratches the surface of the more complicated issues at hand here