Because you don’t really need that many people? It’s possible to have trillions of people and leave all the planets minimally modified and here for future generations to study and play on. Terraforming is enough affront to these planets, dissembling them for a quintillion people who wouldn’t totally end up killing each other and destroy everything is just laughable. And this is coming from someone who works in a story where people routinely dissembles entire planetary systems.