Not just Americans tho. In many parts of the world (China, India, Iran, etc.), you need to go to the top schools so that you can apply to American universities.

Not just Americans tho. In many parts of the world (China, India, Iran, etc.), you need to go to the top schools so that you can apply to American universities. People often do that to get a better life and escape the difficulties of their home country. American universities have so much uncertainty about the quality of applicants from unknown schools, so they reduce their uncertainty by focusing on top schools. I think the same pattern exists in the job market. Firms can often have more "trust" in the quality of Ivy League graduates than other schools.