If you go somewhere else on the planet, e.g. along the Amazon river, you see that the vegetation data reappears, only to start fading again at zoom level 11 or so. It's too irregular and following satellite imagery too closely to have been drawn from human sources such as parcel data. It must have been built from imagery.

So it looks like it's a deliberate choice on Google's part to only use human-derived features such as a park's outline at most zoom levels, on most of the inhabited surface of the planet.