As far as I know, historically it wasn't fixed. People didn't live just anywhere they wanted, they lived where there were exploitable resources, be it farmland, mines, or what have you. If the resources dried up the city simply ceased to be. The idea of creating opportunities where people live can only exist within tertiary and quaternary economies, possibly in secondary. Until about two or three hundred years ago nearly all economy was primary, with a small secondary sector right next to where the resources were extracted (because logistics also wasn't as advanced as what we have today; it didn't make sense to move raw materials very far).