Maybe they've been in space so long that atmospheric flight is problematic for them? That seems unlikely, though. My personal, unsupported belief, is that if t

Maybe they've been in space so long that atmospheric flight is problematic for them? That seems unlikely, though. My personal, unsupported belief, is that if there are aliens, Earth is like a nature preserve, and all the crashed ships are idiot tourists who, despite being told not to touch the buffalo, desperately want to touch the buffalo.