That's just one procedural generation algorithm.
Now suppose I want to take some fruit from the trees. And I want to slice and dice the different fruits. Peeling the skins when appropriate.
Or I want to plant a new tree. Or I want to remove it, with say, a lightsaber.
That's a couple generators more. Now add birds, dogs, insects, moss, moist surfaces, and so on. Hence, my point on needing several generators.