Why can't you "uncastle"? The answer is simple:
Castling is not the movement of a single piece.
You have given the example of moving Nf3, then reversing it with Ng1. This is an example of a single piece (the knight) being moved, then moving the knight again to a legal position. In this case, you are making two moves with one piece (1. Nf3...2. Ng1). With castling, however, you are moving the king and the rook in one move, or the opposite (two pieces with one move), which is why the notation for castling is different than other moves (O-O or O-O-O). With "uncastling", you are moving both pieces which have already been moved. The rules state (in either edition, it doesn't matter) that once the king/rook has been moved, castling (again) is illegal. To "uncastle", you would have to preform the action of "castling" again, which is forbidden by the FIDE rules.
Tl;dr: "Uncastling" is not allowed because it would involve castling again, moving pieces that have already been moved.
Hey !....I have a great idea !....let's call it the Lola Mate. What's the difference ?....it's just as unrelated as Smother Mate, right ?