I agree that there's a high probability of this. No idea how to estimate that probability though. There are far more interesting questions about this than whether it's true or not. Eg did I choose my reality (assuming cogito ergo sum still applies) or was it chosen for me? Does the concept of choice exist in the real world? Or exist, real, world, the,...
This is by far the most likely way for the concept of God to exist. I have long wanted to start a religion based on this. Anyone want to join me?!
Choice is still possible for the inhabitants of a simulated universe to the extent that the simulation builders don't have a second, bigger sim to work out all the alternative trajectories which is then used to control the first sim (and you). If the builders are just as clueless about what happens next (the reason they built the sim in the first place?!?!), then they are hardly in control of the sim and the actions of those within it. And if we inhabit the second sim, then choice is still possible in a simulated-simulated universe to the extent that the simulation-simulation builders don't have a third, even bigger sim to work out all the alternative trajectories which is then used to control the second and first sims. Of course, we don't know if the simulation builders themselves aren't living in a sim.
The presumption of a builder might be flawed, too. There may be no "creators" as such -- perhaps inanimate low-level phenomena naturally lead to simulation/Matrix-like meta structures in any physical universe of sufficient scale, duration, and prerequisite initial conditions.
P.S. Does this religion include St. Turing? That might be cool. Does this religion insist on wearing funny hats and dietary restrictions? That would suck!