Why do developers who could work anywhere flock to the world's most expensive cities?

Politicians and economists lament that certain alpha regions — SF, LA, NYC, Boston, Toronto, London, Paris — attract all the best jobs while becoming repellently expensive, reducing economic mobility and contributing to further bifurcation between haves and have-nots. But why don’t the best jobs move elsewhere? Of course many of them can’t. The average financier in NYC…