What stops us from making this grass?

[Omeletter]

After looking through some meshes in NifSkope, I found out that grass consists of a couple of textured waving planes sticking out of the ground. However, I noticed that some of the meshes, like the underwater grass ones, aren't just flat, they have some sort of a bend. This leaves me wondering, what stops us from making medium/high polygon animated 3D grass, beside much higher requirements?

 

Think something like this. I don't exactly mean like that, but something like it.

http://www.youtube.com/watch?v=8JQ--yA8iQs