My own experience is based on foreign Ph.d graduates from USA universities.
STEM graduates - science, technology, engineering, math.
About 80% of the Ph.d’s from China, India, Russia, and Eastern Europe apply for and receive Green Cards or Citizenship.
The situation is reversed for Japanese, Western Europe, and oil rich Middle East Ph.d’s.
About 80% of them return to their native country.
I assumed that was because the standard of living and employment prospects are as good as or better than in the USA.
If I am mistaken, I apologize.
I can think of only two reasons:
Obviously, people from China, India, Russia prefer to invest into the GC and the US citizenship "just in case." Fortunes of countries change pretty fast (ask Greece.) Having papers allows them to live in the USA and in their native countries, and to travel freely, without visas. Getting US residence papers looks like a very good idea, even if you aren't sure if you will ever live here.