The idea is that if any particular society in a "real" universe is interested in simulating other realities, and if it turns out that it actually is possible to simulate other realities, then we should expect that there'd be a near-infinite number of simulations. Because the society in the "real" universe would likely create many such simulations, and many of those simulated societies would figure out how to create their own simulations, and many of those simulated societies would do the same, so on and so forth. So you'd have "simulations all the way down", so to speak.

Even if there are many "real" universes, you'd still expect each "real" universe to spawn some huge number of simulated universes. Without evidence to the contrary, any individual universe -- including our own -- should not assume it's one of the relatively rare "real" universes. And there'd be no reason to assume we are in one of the "first level" simulations that the "real" universes have created, either.

And even if there are infinite "real" universes, you would still end up with a much larger infinity of simulated universes.