For a long time (since even I was a college student), US Engineering graduate schools have been populated primarily by foreign-born students, particularly from China, Taiwan, and India.
It used to be that such student would typically stay in the US to work. This was controversial, requiring H1B visa, and some allegations of taking American jobs or depressing wages.
Today, these students aren't necessarily sticking around. They're going back to work in their home countries, where they may be more comfortable or sense more opportunity. An example of this is described in EETimes.com - Alarming export: engineers.
Now these very bright students are taking their knowledge and work ethic back home. It's probably a net loss to the US to not have them sticking around. It shows you should be careful what you wish for!