What New York, Boston, Seattle, and Portland have in common isn't that they're big cities, but that they're dense and amenable to non-car transportation. Notice that he didn't mention Los Angeles or Houston, which are still big cities, but which are also relatively sparse monstrosities that are only navigable by car.

> Further, oddly enough, there are lots of people, of all ages, who don't live in New York, etc. Why don't they move? Maybe because they don't want to.

You're missing the point. A good way to measure the desirability of a given location is to compare rent, because rent literally measures the market value of living in that location. It turns out the market value of living in Seattle, Portland, New York, or Boston is pretty high, which is the free market's way of telling us that people want to live there.

I'll stick up for Houston a little bit. Yes, public transportation sucks and you need a car. But the near-complete lack of zoning means that most of the places you need to go are relatively nearby, so if you live close to work (usually possible because as you note real estate is cheap), you won't be spending that much time in traffic.