In this context, why do we think there would be many other civilisations? In the context of the Fermi paradox we think there would be because the universe is so

In this context, why do we think there would be many other civilisations? In the context of the Fermi paradox we think there would be because the universe is so huge and we can calculate that there would be lots of planets capable of supporting life. That said, the structure of our universe is irrelevant if we're considering whether it is a simulation. The "billions" number that applies in the case of the Fermi paradox doesn't apply to this conversation. I'm also wondering where it comes from.