Why does it seem likely that we're in an ancestor simulation rather than being in a simulation of some other sort? We could be in a game, or a non-historical educational tool, or a big hobby project. Or, of course, in something which doesn't correspond to any of our motivations.
We create a huge number of simulations in fiction and history and gaming. Some of them are more or less accurate, many of them aren't. Our simulations do tend to resemble us, or at least our models of what's interesting about us, but I'd assume that a more advanced civilization would have a wider range of invention.
By the time I saw 'The Matrix', after all the hype about how mind bending it was, I was disappointed that the Earth turned out to even be real.
Today's Philosophy Bites features Nick Bostrom on the Simulation Argument. Here is a direct link to the MP3 file.