[altereds]

I see this as a chicken and egg situation. Even within any country if you see the salaries are higher in cities where you also have most job opportunities. This attracts workers who seek more opportunities and higher rewards. Those who are not able to compete or not interested to compete would not migrate to the city. Companies that operate in these places have to offer more to get the best among those available. Companies that need to get the best talent have to be ready to bear these higher costs to operate from these cities.

Reading up on the history of California, I see from a long time back since the times of the gold rush it has been a place which attracted people seeking more opportunities and the best people would stay there when they are rewarded for moving there. There may be multiple such cities and locations in the US , but even within the US salaries would differ in different cities.

I think the best answer may have to come from an economist who would understand these better.