Why so many Indians work in US/Australia instead of India?

I presume salaries are considerably better, you have much better direct access to prominent western companies (FAANG etc.) without needing to go through third party contracting shops, you also have better access to western education, population density is lower, average living space per person is higher, you have an opportunity to potentially escape the caste system if that's relevant and quality of life (pollution, opportunities, etc.) may, may seem to, or may eventually be better.

There is nothing unusual in this.

The market for highly skilled people is global and people will prefer to work in places with higher wages, better living conditions and quality of life.

English education and familiarity with democratic political system makes the US, Britain, Singapore, Australia and other countries relatively more attractive than other countries at the same income level.

The growth of my learning and experience was too slow while in India. I was unlucky but after coming here I learned hell of a lot about software engineering and infra stuff. Money is good but only after you go back to India and while you are living here the standard of living is very very poor as compared to when I used to earn in India. But as long as I get to learn and contribute and get experience in what I wanted I am all good here. I want to go back but only when I know that I have a better opportunity back than here. I miss my country too. The culture is here is just too weird with all the shaming and the tipping culture. People get hurt with the slightest thing here in US which is so so wierd.

Actually prestige is simply an euphemism for purchasing power. Just because you may get a better living quality outside US doesn't mean that that's the better option unless of course one's food rent alone is all that you ever care about, for prices in general are fixed across borders, increasingly a problem as borders are getting nearer. Like, you may think that martians got nothing to do with you since they are far, but as sure as globalization, then-they-came will come.

Tldr: money makes the world go round / no man is in a well.

Add to that list Europe, Middle-East, Japan and South-East Asia.

Interestingly, yesterday I was watching[0] an Indian journalist analyse the Lancet report related to effects of population growth on various countries. He was analysing it from India's perspective. None the less it is very interesting analysis and provides some answers to your question.

[0] https://www.youtube.com/watch?v=U5ariMoaxCk

Are there really that many as a fraction of total Indian population ?

Exactly how can there be any other answer other than $?