Constants are the simplest assumption. It's a "first round" of modeling that lets you start having philosophical thoughts on the implications. You can always in

Constants are the simplest assumption. It's a "first round" of modeling that lets you start having philosophical thoughts on the implications. You can always increase the complexity of the model after. If you don't hold some parameters constant, the model quickly becomes too complex to be of much use for philosophical questions. In reality, a civilization can expand with exponential acceleration, deceleration, or shrink. I didn't even think this the worst of the assumptions. The fact that expansion stops as soon as a civilization meets another seems silly too. As we know, civilizations love nothing more than respecting borders and not conquering each other. But again, it can go either way, so they chose the middle assumption.