OK, since you insist on reasoning out Levitate -- riddle me this: How can levitate make a BOOMERANG MADE OUT OF BONE, do absolutely no damage? Last time I checked, boomerangs are actually used to kill things that are in the air. And while you are coming up with that reason, answer this one too: Why would a levitating pokemon not be hurt at all, if you smack it in the head with a BONE CLUB?Sure, you can use the whole "Levitate" doesn't mean you should avoid mud. But mud (Mud Bomb, etc.) isn't really a "ground" thing, but it's not necessarily water, and it belongs to the dirt. If a truck was driving through the mud, it would not necessarily make a huge "splash" and "flying things" would not necessarily be.. hit by the mud. This reasonable to you guys?
Give up. Your argument is fail. There is no logical explanation for every move associated with a type immunity. You give immunity based on the general theme of the move type. In that sense, Levitation grants immunity to ground. By the same token, Mountaineer grants immunity to rock.