It's an issue of performance. Foliage is difficult stuff to render and CryEngine aside, pretty much all game engines struggle to do it, so they fake it instead. Unreal is no different. The fancy jungle UDK level looks great, but when you load it up you discover that realtime performance is terrible and the total size of the map is barely any bigger than a suburban backyard.
We'll be doing what we can, but don't expect any maps to have large areas filled only with thick foliage. Just because you can't see through it doesn't mean that the engine can't and nobody wants to play a slideshow.