Because people underestimate the nature of modern urban clustering. Brooklyn has decent average household earnings, as you'd expect of a relatively hip, expensive, (somewhat) nice place. But centers of business and culture absorb as much as possible from the areas that surround them. So a huge percentage of people in Brooklyn work in Manhattan, particularly higher-income workers.
So when people say the UK has the most to gain from the EU are probably much, much more right than people are guessing. Talent from an entire continent is pushed into the funnel of London, with only absurd commute times and low enough wages to no longer tempt the talent being the only things preventing all people living in one giant megacity.
Don't get me wrong, that would probably have terrible quality of life issues, but Manhattan is a literally terrifying place, and it hasn't stopped it from gobbling Brooklyn whole, despite all the culture and livability there (comparatively, of course).
In general, we ought to model all countries as talent-sinks, major cities people move to for work, and talent-sources, places where people might live and get trained before being put in a pipeline with relatively compelling incentives to clump into a talent-sink.
We should remember the talent-sinks are tiny – to the point that "New York City" isn't specific enough, and remember that talent-sources have, in equilibrium, literally every advantage not related to business. If Manhattan was a great place for families, it would just attract even more high-level talent, until the cost reflected "good for families"+"good for career" value. If you aren't looking for a bustling talent-sink, you'd anticipate it being a terrible return on investment to live there.
It's probably best to view the societal effects that stem from this in multiple layers, and people self-select into groups defined by those: not in a talent-sink; working-but-not-living in a regional talent-sink (like Columbus, Ohio – the more normal cities); living in a regional talent-sink; working-but-not-living in a national talent-sink; living in a national talent-sink. Those groups have different values that drove them to be in those places, different outlooks, and different life trajectories. It's worth distinguishing between them.