Areas with lots of natural leaf-fall are rich with decomposer organisms (fungus, worms, "bugs", slugs, etc.), so leaves are quickly consumed and turned into humus.
Many human-contrived environments (lawns, cultivated ground, etc.) have little of those so the leaves stick around much longer. I'm active in a community garden, and many of the plots are mulched with leaves in the fall, and lots of those leaf piles look exactly like they did when laid down six months ago. And some of those plots have no leaves left because they have all been blown hither and yon by the wind.