system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do some scientists believe that we are living in a simulation?
gpt
We think it should be possible to create convincing simulations of our universe within our universe using computers. Why, then, must we be at the "top" of the simulation stack? When you consider that any given "real" universe could contain any number of simulations, and that each simulation capable of supporting computation could also support any number of simulations, you suddenly realize that simulation universes become a ton more likely to be chosen at random from the universe pile. 

It also explains some of the weirdness surrounding quantum physics. Why does observing particles seem to give them a history they didn't have before? Because our observation forces the simulation to update the particles and make them more than a probability field for us to observe!