Why wouldn't the fall change our bodies. Didn't God say if you eat of it you will surely die? There are hundreds of other passages both in the new and old testament that say we are disease ridden evil individuals.
Anyway, you asked why some people might think humans are perfect, and wnoise answered why some people who read the bible might reasonably make that assumption. What exactly is your continued issue here?
Well, I mean they are looking for fact in a thousands year old collection of fairy tales by assorted authors, but they're not particularly any more willingly lazy or stupid...