IMHO couple of reasons:
1) Extreme social distancing. Cops there are actually caning people who violate stay-at-home orders.
2) India is awash with Malaria and those who can afford to do so typically take HCQ as a prophylactic.
#1 is curious as at any given time there are at least fifteen diseases circulating in India that are worse than this one.
You make a good point. Even in the worst parts of India they are exposed to some nasty stuff from birth.
Poor immune systems would not last long in the environment.
Check out these videos and your statement on social distancing.
https://www.youtube.com/watch?v=oUs88p8wtBM
https://www.youtube.com/watch?v=jv8qwg0bJC0&t=2753s
Then think tent dwellers in LA. They are the beginning of the Mumbai slums