alt Hacker NewsCountries subsidize rural living because it enforces their control over the frontier.
The United States is difficult to invade because of the oceans surrounding it and the many people with guns in the interior that'll take shots at armies.
If you put everyone in a few cities on the coast, the USA becomes easier to invade.
No country capable of landing a single troop on the lower 48 is scared of undisciplined men with AR-15s.
In fact I am not sure if any country can get a troop transport near the US coast without being nuked to the ground first.