Link's FS was never in any game either, but that doesn't make it any less of a badass move!
I don't think every character's FS will be based on an actual move in their respective games because not all characters ever had "game-ending" moves. Mario, for example, never really had an attack in any game that screamed "holy moly, that was powerful!" Therefore, his FS is based on what he does best - shoot lots of fireballs - and take it up a couple of notches on the power scale.
On the other hand, Pit's FS is based on his summoning of the angelic soldiers that he saved in the dungeon prior to the boss fight in that dungeon. If you ever played Kid Icarus, you know how invaluable those soldiers were in those boss fights. That's why it's a FS (IMO).
I'm sure as more older characters are introduced we'll see more FS's that are unique to Brawl, but that's all good, so long as the FS fully represents that character.
~Old Man Nintendo