Many factors account for this:
i) Curriculum that subjects them to go and explore the high paying tech fields (math, engineering, science) arent as challenging as they are abroad, especially in Asian counties. Furthermore, interest in math/engineering is low; you might remember Obama bring that up in his last presidential campaign speech.
ii) The foreign students who attend college here pay almost double of what an in-state student pays. Furthermore, federal financial aid to them is not an option, along with very few minuscule scholarships. For them, its a do or die situation, they HAVE to work hard and make that dollar (converted from Rupees) that goes towards their education really count so they work harder.