We are talking about probabilities, not certainties. They might have been able to drive 45 yards to FG range. If they saw the ball again, if the Cardinals didn't score a TD, if the Cards turned it over, if the Packers could beat their 38.6 yards per drive in the game. Incidentally, before the Janis dual hail marys, that figure was 33.3 yards.
But none of that is as likely as scoring from the 2.5 yard line.