> Why is there a billion people in India if these conditions exist?

Not to be crude, but this should be obvious. Similar to many poor areas anywhere in the world, but what else do you do all day hiding indoors (due to violence, poverty, et al) for entertainment, with very limited income? Physical intimacy is fun.

It's also partly because the society is agrarian by population, culture, and education. This is born out by behavioral signals "Despite high awareness (over 99%), access and choice remain challenges, with only 56.5% of married women using modern contraception according to NFHS-5 (2019-21)" - Children are a support system and a labor font. Growing wealth by growing a family is a historically successful pattern (albeit not a guarantee).

> Its never made sense to me why the birthrate was so high in countries with these hostile conditions, meanwhile the population in the United States with excellent conditions is declining.

What do humans do all day with all the creature comforts and an expectation they will always have them to some degree? They have less children in trade for better income, investment, and comfort. Industrial nations have been tracking this for decades.