Simple explanation: Wilcox County, Alabama and Buffalo County, South Dakota don't attract many visitors from overseas — but those counties are included in the statistics, along with similar areas of the USA that resemble the second world or third world.
Nations of western Europe have had their wars and genocides, but in terms of statistics two things are in their favor. It was 2000 years ago, not 200, when the Romans subjugated the indigenous tribes of western Europe; and during the three centuries of active slave trading, western Europeans ensured that slaves went to their colonies and not their own backyards. In contrast, Americans still deal with the after-effects of both the Trail of Tears and slavery. To a large extent, deprivation in the USA today is concentrated in African-Americans and native Americans. It's not their fault that they're poor. It's the fault of the rest of us, and doubly so: our ancestors forcibly put African-Americans and native Americans into poverty, and for the most part we have failed to lift them out of it. Some white Americans even blame African-Americans and native Americans for not escaping poverty somehow!
I don't know as much about the holocaust of native Americans as I know about slavery. From 1525 on, roughly 12 million Africans were seized and put on ships to the New World as slaves. About 2 million of them died at sea because of atrocious conditions, abuse, or outright murder (in the latter days of the slave trade when some European navies were patrolling for slave ships that had been outlawed, the captains of slave ships would simply toss their human cargo overboard when challenged). Brazil took 5 million of the survivors. About half a million went to what is now the USA, directly or indirectly, mostly in the 18th century although some slaves were brought in subsequent to the USA's Act Prohibiting Importation of Slaves of 1807. Millions more went to Caribbean islands, where tourism by wealthy Americans and Europeans has not counteracted the consequences except in isolated spots.
I've seen the favelas in Rio de Janeiro from a distance, and I've seen the interior of Puerto Rico. They reminded me of Wilcox County.
Did my ancestors in South Carolina and Alabama own slaves? I have never heard or read anything that indicates so. To the contrary, all available information says my ancestors were subsistence farmers, not members of the wealthy class that owned plantations. However, when I think of current statistics such as infant mortality, personally I remain thoroughly indicted by leaving undone those things which we ought to have done.
As for American braggadocio such as "the best county in the world", I hear the phrase most often from Americans who have never actually lived anywhere else, except in military service — and who certainly haven't lived an impoverished life in Wilcox County, Ala.or Buffalo County, S.D.