I see it as they were simply created in response to anything that would threaten the planets destruction, but it just happens that at the time it was Jenova. Had something/someone else come along by then with the same cataclysmic undertones then they would have been there in response to that. They don't awaken just b/c Jenova's around, they awakened b/c they sensed what trouble Sephiroth was bringing to the planet.