I'm not convinced it's not the Internet. Google "FOMO" for a possible mechanism. The Internet makes it easy for large cities, by sheer social mass, to send out a louder cultural signal and convince everyone else they are missing something by not being there. Talk to a young midwesterner and you will hear many whose ambition is to "get to NY," with this vague sense that magic will happen when they do. This draws more young ambitious people, which amplifies the effect. Eventually if you are not in SF or NYC you are nobody.
This isn't new, but I do wonder if the net is amplifying it.
I also wonder if wealth stratification is playing a role. Opportunity clusters in cities where there is capital available, and those tend to be the big cities where the most rich people live.