As a country, we deeply suck at moving humans around. Europe has actually fast trains and they're actually reasonably priced, unlike Amtrak. If you want great service in Europe, you take Virgin; if you want cheap flights, you take RyanAir. We lose by getting RyanAir service for Virgin/BA prices. We're great at shipping bits, U.S. freight rail is world-class, and we can get almost any fruit in any season, but when it comes to passenger transportation, we essentially stagnated in 1949. Air travel is still expensive and far more unpleasant than it ever was, passenger rail is at the same speed it had in the 1920s, and most of us still rely on the car for commuting. If you want to live in a place where you can actually walk or bike to a city, you'll pay absurdly high prices to live there. This mediocrity in transportation is a deep national morale problem that is exacerbated by the current economic climate (i.e. many people having no fucking money). A result of this is that the world is becoming larger. Much larger.
Moreover, what's left of the job market fills through social connections. "Front door" approaches are officially dead. It's possible to make "weak ties" and acquaintanceships online, and that kind of "networking" is often enough (strength of weak ties) but if you fall into a bad spell (or, say, are female and take a few years off from work to raise children) and need someone to take a chance on you, you're going to want strong ties. With wealth disparities widening and talent geography becoming more unfavorable, people are less inclined to believe they'll easily make strong ties after college; it rarely happens. So people are averse to moving without a strong reason to do so.
Then there is the difference in job markets between continuous (efficient, low-latency) and discrete (illiquid, high-latency) ones. For high-talent jobs and candidates, there are 3 places where the markets are continuous: New York, Silicon Valley, and Boston. That's not to say there aren't great jobs (and great people to hire) in places like Portland or Minneapolis. There are, but there isn't the massive number that makes a market continuous. A lot of talent is coalescing in these 3 areas because living in one of them means that changing jobs doesn't mean that one is likely to be changing cities. What that means is that people who are willing to move for economic reasons, by and large, have ended up in one of those three places (and will stay there).