Carrying an empty plastic bottle / food packaging to the nearest litter bin (or even home if there are no public bins) costs no money and very little energy. I do this all the time even when I'm very tired. From what I see throwing litter around in many countries is socially accepted behavior. It's not like they have no choice and more like they don't see it being wrong.

I do see a correlation between amount of litter on the ground and country economical/political development but cannot think of a convincing casual link here.