Time travel. If time travel was possible, then presumably someone from the future would have already gone back in time to change the past. Therefore, when someone says they, for example, would have stopped Hitler, they actually wouldn’t because someone already would have made that correction in time. Instead, that must have been, unfortunately, the best possible outcome out of all possible outcomes. Either that or time travel just isn’t possible which seems significantly more likely.

–u/izackthegreat