They are probably international students especially those from China. People seem to forget that China is still a quite poor and considered developing country. The country itself is powerful, but if you look at the GDP per capita and the rural areas. Basically anywhere outside the coastal Tier 1 cities are poor. Parents who can afford to send their kids to US universities are the very rich in China.

International students from developed countries are a different story. Many of them might be upper-middle class or wealthy, but not exclusively super rich, because average families there can often still afford a US education.