Both for performance reasons and realism reasons: I think the forests are too thick. They could be thinned by 25-50%, and look just as natural if not more, and I assume would relieve our GPUs some work. Some forests, mostly tropical and temperate rainforests and high altitude alpine forests are the only ones naturally as thick as in the simulator. I also figure less instances would let us increase draw distance without suffering performance loss.
This is coming from someone who works in forestry in the American west.