It should also be highlighted again that the First is a special case.
First, as far as we know, a Flood happens when there is a sudden, dramatic shift in the primordial forces of Light and Darkness. A world can be Light-heavy or Darkness-heavy and still find a tenuous equilibrium if it's left alone.
A Calamity and Rejoining happens because of instability involving elemental forces, not necessarily primordial forces (though imbalance in primordial forces can make it a risky venture). Calamities are elemental. Floods are primordial. But here's the thing: the primordial forces affect the elements. In the presence of Darkness, the elements will become more active, gaining an "astral" charge. In the presence of Light, elements will become more passive, gaining an "umbral" charge. Because astral and umbral are elemental in nature, they can be harnessed for Calamities/Rejoinings. Because the First was so unstable and Light-heavy, it made sense to try to manipulate the Light to skew the elements umbral until the conditions for a Calamity were achieved. That incidentally risked a Flood.
But nobody wants a Flood. And if you're on a world where you can imbalance the aether towards a specific elemental force without worrying about primordial forces all that much, you might not even run as much risk of a Flood, for all we know.
For more information about elemental vs primordial forces, I made this infographic to be a smart-ass but it got passed around a fair amount so I must have struck a chord, lol.