I wonder why? Pokémon and Fantasy Life have one.
Fantasy Life's looks like it requires a flashcard or such. If any developers are interested though, save editing should be possible for almost every game now through web injection (Pokemon's method).
Nope. Fantasy Life's is interesting, since all of the main save data is stored as 'extra data', which is inherently on the SD card. You just use the extData dumping tool, and you get access to the save files. Likewise, when you own a retail cartridge of the game, the save file isn't connected to the cartridge. I'm guessing this is why an editor popped up so quick.