Why aren't obese people healthy? they have so much (cellular) wealth stored up and such big organs.
Ir's not that different. If you concentrate economic wealth in cites, the periphery starts to suffer economically because it can't exploit local resources effectively, and the population starts selecting for city activity. Over generations, that leaves you vulnerable agriculturally and militarily. If you experience a natural or military disaster that deals a massive blow to one of you key cities, the whole country can collapse.
Ir's not that different. If you concentrate economic wealth in cites, the periphery starts to suffer economically because it can't exploit local resources effectively, and the population starts selecting for city activity. Over generations, that leaves you vulnerable agriculturally and militarily. If you experience a natural or military disaster that deals a massive blow to one of you key cities, the whole country can collapse.