Other than dumping tree branch prunings in the forest, nobody leaves garbage in the forest, it's too far to drive when the transfer station is both nearby and free.
The problem here is that it's free for householders but costs business to legally dispose of rubbish. So, if a builder does up a kitchen, the waste gets fly-tipped (not all builders obviously). There are gangs that will go around offering to remove rubbish, take a fee, then dump it on public or even private land.