odds are, you didn't happen to be born in the optimal location for your desired career. if you can get paid $20k more on the other side of the country and a flight home costs $200, you might find it pretty tempting to move. if the flight cost $2000 or you had to drive for three days to get home, it would be a lot less appealing for people who value seeing their family.

I don't understand how it's so hard to see that cheap flights make a big difference at the margin.

In fact - the drastic rise of living costs in economic centers indicates that people will take that move, even if that trip back is several hundred dollars and takes many hours.