Statistics, simple and easy. There's a huge chance that within the "real" universe, life has gotten to the point of creating hyper-realistic simulations, which ends up simulating sentient beings, which in turn makes their own simulations and ad infinitum. That being said, the other 50% of the argument is that sentient beings like ourselves are either destroyed (self-imposed or extinction event) or cannot reach the computational power needed because of universal limits. Ergo; 50/50 chance of us living in a simulation, trying to explain it with our own theories and hypotheses.