I dont think this has anyting to do with unability of the brain to calculate the scenery. It's more of a "the more experienced you become, the better it goes"-thingy. I've had several WILD's where I had problems with vision, it still happens from time to time. What I do is just ignore it and try to walk around, rub my hands to stay in the LD, and at some point often visuals will return. It might also have something to do with the fact that your brain still has to switch the internal "dreaming machine" on when you WILD (Not during DILD's because you are already deeply in a dreamstate most often). Scientificaly speaking this is the secondary visual cortex, which creates dream images.