Education is only one factor, there are many others.
FYI - 'Worker mobility' in the US is actually declining, and has been for some time.
This could possibly mean that regular workers are moving around less, relative to educated one's.
It also could mean as communications and transportation become easier, the 'big city' advantages are just compounding.
So the major metro areas are just sucking up everything around them.
Texas, Colorado, Illinois have a sea of purple around them.