Those of us who work in public landscapes find litter in the strangest of places. It gets everywhere. It is not necessarily put everywhere. It just has a sneaky way of getting everywhere. By nature, litter blows about and gets washed into creeks and rivers that flow out to the bays and oceans.
Of course, there is much more litter in public spaces with the most traffic, such as city parks, than there is out in remote places where fewer people go, such as hiking trails in state parks. People are not necessarily slobs, and most put their trash into the appropriate receptacles. There just happens to be more litter where people are because that is where most of the trash that becomes litter happens to originate. Most litter that accumulates on the sides of roadways was blown there from the open beds of pickups. Not much is discarded out there intentionally.
One of the projects where I work is designing trash receptacles that wildlife can not get into. Racoons, which some of us know as ‘trash pandas’, are notorious for distributing large volumes of trash into the forest. Squirrels tend to be a bit more selective in taking mostly biodegradable bits of discarded fruit, and by unwrapping their finds before taking them away. Crows are actually worse than squirrels because they will take larger bits of trash merely because they find it to be amusing. Once out of the receptacles designed to contain it, trash gets blown about by the wind. In fact, wind alone can blow trash out of some types of trash receptacles, such as those fancy cylindrical steel mesh receptacles that suspend trash bags within, like those that are so common downtown in many cities. Litter is naturally an unnatural consequence of modern civilized society.