Why do most Indians go abroad for research while we have some premium institutions in the world?

When you compare the number of premium institutions in India to the number of research experts in the US or Europe, you’ll notice that India has a very low ratio.
Because the number of premium institutions (IISc, IISER, TIFR, NCBS, and so on) in India is extremely little in comparison to those overseas.

Given India’s population, the number of reputable schools available to enroll skilled professionals is insufficient, resulting in fierce rivalry, as is the case with every opportunity in India.
So many good researchers go abroad, not because they aren’t good enough, but because there are more possibilities and a more meritocratic selection process.