Probably because thats where the most people are.

No, they're often less dense and smaller than residential areas, and more walkable, but they attract people who don't give a shit because they're in a mindset where they don't give a shit. They require a lot more cleaning too.

When you go to Asia

Already there. Also you should be more specific, EA and even Central Asian countries are reasonably clean. When you're talking about places with monumental amounts of trash, you mean SEA and India, and even there, lots of places aren't like that.

Places that are a bit more affluent tend to have the resources to dedicate to keeping the streets clean. Westerners on vacation don't have an excuse.

The Japanese are better than the westerners

Maybe, in cities, but there's a bit of a littering problem in more rural areas and in rivers. Also in the poorer areas of cities.