[bnug]

I think its because advanced degrees fast track the immigration process, and the better paying jobs are in the US. Those born in the US get a bachelors degree and then a job (probably to pay off the student debt). For a non-US born person with a bachelors degree, it is far more difficult to find a job in the US, so they apply for a MS/PhD program. Then the MS/PhD programs are full of foreign nationals, and naturally the academic R&D jobs pull from that pool made up of mostly non-US persons.