One word: Culture
But since America is the richest country on earth, your logic would suggest that we have the best culture in the world.
You can be expecting a visit from the Multiculturalism PC Gestapo at any moment!
Two words : Free Markets. Indeed, people in North Korea are starving right now, even though they have exactly the same culture and they're the same ethnic group. Not to mention the fact that until the 80's Mexico was richer than South Korea.
Ireland, Spain,Portugal,Greece ,Singapore and Taiwan became rich countries in the last 30-10 years without having to change their cultures