America is quite unique in the world by being so significantly multicultural, and increasingly so. It can tune and balance its demographics with immigration. It is especially good at attracting the best and brightest from around the world, because there is a realistic, and well trodden path, of not only fully integrating yourself and your children into American society, but also integrating into the ruling/upper class.
For example, any Chinese citizen can effectively defect and build a safe and sustainable life in America. Millions have already done it. First generation immigrants lead some of America's most powerful corporations. Second generation immigrants are welcome as officials in the government. The current VP is second generation immigrant of Indian/Jamaican descent.
On the other hand, an American can never effectively defect to China. Maybe if you are ethnically Chinese, your children could be accepted, but not likely anyone else. China simply doesn't have a culture of fully integrating people of all kinds into it's society.
The funnel of exceptional humans is one way, and it's into America.