Because both players have already taken two turns, a player can claim a draw after only 48 moves. If the movement of the knights was repeated 25 times, for instance, then either player could claim a draw. However, this is not possible at the beginning of the game, so simply moving the knights back does not actually restore the game state.
It's just built into the rules: 50 moves without a capture results in a stalemate (https://en.wikipedia.org/wiki/Fifty-move_rule). I guess you could say that the two moves 'go' into the implicit state counter that keeps track of the number of moves without capture.
Because both players have already taken two turns, a player can claim a draw after only 48 moves. If the movement of the knights was repeated 25 times, for instance, then either player could claim a draw. However, this is not possible at the beginning of the game, so simply moving the knights back does not actually restore the game state.