Arguably a complete simulation of all physics is easier than a simulation that has detailed memories of last week (plus fossils and microwave background radiation) without actually simulating last week/evolution/Big Bang. In other words for similar reasons that we can stop worrying that God just created us just now (or that you are all just dream people etc.) we can stop worrying that Wednesday didn’t happen. In some Maths sense if Wednesday had to be fully calculated for today to be rendered then it happened.