As you may agree that most departments of most US university will have a number of Indian postdocs. But why very few Indian professors leave India to join US universities. I wonder why?
I think the answer is likely to be much the same reason that you see many postdocs shifting between the US and Europe, but few faculty changing shifting between the US and Europe as well. Postdocs are in a transient period of their career and will readily move to follow opportunity. Faculty, especially once tenured, are likely to be settled and not particularly interested in moving, no matter where they started.
Now, India is still different than the US academically, but as India continues to develop and Indian academia continues to expand, the difference is decreasing and one should expect the flow of scientific talent to continue to become more balanced, as is happening already.
In addition to jakebeal's answer, I would like to add some points of my own. India is a developing country with not much money to spend on funding. Thus only a handful research institutes can support good quality research and education. Talented students who don't find enough resources in India go to abroad to pursue their endeavours. The people who remain in the country are the ones' having a 'sense of patriotism' and are here to serve the country. Hence, they refrain from leaving.
You will find most of the Nobel laureates and Turning award winners from India being non-residing Indians.