No one said America was problem free, sin-free, or has a spotless history of human behavior.
You have to open your eyes and realize the problem of sin and evil inflicts every human and every nation. It is a universal problem across time and across cultures and across the globe.
America is the *best* country in the world, compared to every other country.
If you know of a better country, name it. If you know of a better country, what's stopping you from moving to it? America actually lets its citizens leave if they want to. Many countries don't.
North Americans have a lot to be thankful for, especially the amount of freedom and opportunity enjoyed here.