I see many Indian researchers going to the USA as postdocs. But I see very few Indian faculty leaving India. Why? 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.