And I totally agree - I think western suburban individualism/isolationism is a terrible idea, and horribly corrosive to peoples long term happiness and health.
You're certainly free to choose a high-density lifestyle for yourself if you prefer it.
Very little of our evolutionary history occured in high-density urban environments, so it seems extremely far-fetched (bordering on ridiculous) that it would somehow be "healthier" from a psychological perspective.
Oh wait.
That doesn't sound right at all does it?
No, it doesn't, but you're shifting the burden of proof. Antibiotics have been demonstrated to improve health. Someone who claims that some new stuff called an "antibiotic" improves health is expected to prove it, just as the burden of proof falls on the OP here to show that the "new stuff" of living in a city is actually healthier.
Urban life brings stress, high crime, bad education, pollution, traffic accidents, and many other factors that demonstrably either reduce the quality of life or endanger it outright. It's fun to live in the city if you're a young single, but not particularly "healthy". It's not at all good for children.
If you have some evidence that urban life is actually "healthy" in a psychological sense, let's see it. Do you have any?
Those people came there for jobs (i.e., survival). Now we don't have to do that, and the population is flowing the other way.