Google does have vegetation and desert data, but you can see that it starts to fade around zoom level 5 in the US (at which you can only see a number of entire states and the names of major cities).
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.
I used to work for Google, but not on maps. You can see the zoom level for yourself in the URL. Use the scrollwheel or its trackpad equivalent and the Z parameter in the address bar will change. That's where I got the numbers, not any special knowledge.
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.