The laws of physics we observe have far to much unnecessary complexity at the small scale to be worthy simulating unless it mirrors the simulators reality, in which case it’s unlikely that you could simulate earth on anything less than an truly absurd collection of matrioshka brains, and even then it would be to expensive to do often.
Moreover, what would actually be the point of an ancestor simulation in the first place? Things are going to diverge so rapidly that the only things you can learn are very general statistics, which you could make good estamates of already and with far less computing power. These are also statistics that self evidently do not matter becuse if they did occur often enough to matter than the simulators would already know just by looking around their own universe.
Basically the only things i’ve heard that don’t require an intelligent civilization that could reach technology far in advance of our own but is comprised completely of people so psychopathic they’d create billions of children just to kill them off for mad science are things like your parents wanting to raise thier children in a simulacrum to the distant past or wiping your own memory for some roleplay or similar. In that case, where you are only simulateing a few rea people and a bunch of NPCs, why would you bother designing NPCs to being up the simulation in the first place?
Foreshadowing and getting you used to the idea maybe, but there are a lot better ways to do that, most of which involve an actual conversation.