Talk to some expat Indians and you may get an answer.
As an expat, my reason is simple. Apathy!
In America, you are very likely to be admonished by a stranger for littering in a public place. Trash cans are everywhere, and the laws/regulations are enforced, so the risk-reward of littering/pollution is overwhelmingly in favor of proper disposal. Most of the population doesn't have to work 14-16 hour days, 6-7 days a week, so there's a lot of volunteered time to go around for maintaining public areas. There is public funding and enforced regulations to handle waste management, and industry employees have union representation.
India is diametrically opposite to this - it would be risky to admonish someone for littering, especially if they look wealthy, because rich Indians are usually above the law and consider themselves so. Everyone bribes the police, so anyone can pollute the environment for a price. In everyday life, the tragedy of the commons is certainly prevalent - there's already a pile of trash, so one more wrapper makes no difference. But there is also apathy in the sense that you're not going to admonish the person dumping a rickshaw full of trash into the river, because that's the only realistic way for him to feed his family, and it's probably the only economical option over dumping it somewhere yourself. And there isn't much evidence that paying more in taxes is going to do anything for the problem, and a lot of evidence that it will just line your local politician's pockets. The people who actually do the work are usually the poorest in society, and they don't know the dangers of lighting carcinogenic materials on fire.
To summarize with an anecdote - I saw my uncle throw an empty chip bag on a beautiful beach in Goa, so I picked it up and asked him why he couldn't just hold on to it for a while. He shouted back, "it's someone else's job to pick that up!"
Counterpoint: In Japan, I often had to haul my garbage quite a bit before I found a trash can to dispose of it. No litter though. (To be fair, there are other factors; most people don't eat on the go, and there are usually garbage cans near convenience stores and vending machines).
I'm definitely not the most well-meaning person, but that piece of trash is going to go into my pocket (or stay in my hands) until I see a sufficiently-empty trash can or dumpster to place it in. It'll stay with me until I get home if it has to.
I _do not_ get folks that toss trash around. It's just fuckin nuts.
13of40 is implying that that trash will not end up in a trash can, but will end up on the ground, because they believe that even the most well-meaning person will refuse to carry refuse with them until they locate a trash can. (I -and others- have direct evidence that supports disagreement with their bleak outlook.)
Aside from the occasional (American) football or baseball hooligan tipping over a can in "celebration", trash cans in my area get emptied into the trash-disposal truck that stops by on a regular schedule. It's legitimately regrettable that you live in a place where your trash cans get emptied into rivers or other common areas.
If what you're telling me is a true and accurate picture of the state of waste management in your area, then that's a crying shame. I sincerely hope that wherever you are, you and your compatriots a successfully able to convince enough people that the commons is worth protecting and that proper disposal of trash and other refuse is worth doing to change this sad state of affairs where you live.
I think it is not mere apathy but learned apathy. There is no way for the people to know who to hold accountable. The political structure is a mess. It's not even googleable.
In addition, most people don't know that they need to constantly harass their representative to get things done.
Lastly, after many decades of political decay, most rich people only clean up their high rise societies. No one fights for public streets.
That's why there are few traffic lights, no lanes on streets, no crosswalks, ruthless traffic etc. People need up-to-date websites with contact details of elected representatives so that the representatives can be spammed to death about their inaction.