Cause US is the largest and the most superior on so many indices economicaly, sometimes providing the only top notch opportunities in the whole world (e.g., advanced high-tech). However, lots of hard working people from poorer or less developed parts of the world immigrate to countries other than the US in large numbers (legally, for education and skilled work). Most people in African countries (North Africa in particular, but also subsaharan, with the exception of South Africa) immigrate to Europe, namely France, Germany and Sweden. And immigration is just as successful as in the US (there are lately immigration issues in Europe, but it's a different type of immigration, refugees and asylum seekers, caused by destabilised regions in the middle east. I'm talking about regular immigration for education and skileld work).
