The why of the simulation is an interesting question. One hypothesis:

+ NIs and AIs will always try to find ways to break through glass ceilings of intelligence. They are driven by a never fulfilled thirst for knowledge and engrained with a curiosity instinct.

+ An AI will immediately run simulations and set breakpoints on processes that undergo complexity transformations.

In other words, we might almost have been spotted. As soon as we build silicon AI the simulation will stop.

It explains why we have not been contacted by aliens too. :-)