In the Doctor Who episode "Extremis" we learn that
The Earth is simulated by a race of aliens, (refered later as "The Monks"). That this truth had been discovered centuries earlier and put in a book, The Veritas, written in a forgotten language that has just been translated
The Veritas contain informations on the limitations of the simulations, and can proove that people are simulated
Thus being said, the Monks have no interest in letting the Veritas exist in the simulation, actually they would get a much more representative simulation if the limitations of the simulation were kept secret.
Moreover, we see them directly influence the simulation, by attacking people. So why didn't they destroyed the Veritas centuries ago? Thus keeping the simulation more realistic.
Related question here.