I'm not sure why you say that. Does pain have some property that makes it impossible to simulate?
If an organic animal can feel pain and it is possible to create a 100% accurate computational simulation of that animal, then the simulated animal must also experience pain. To me the interesting questions are at what level of simulation fidelity <100% (a) does the simulated pain be said to be "real", and (b) does it make sense for the simulation to be said to give rise to a subjective experience (if at all)? And the follow-on questions for both are "how?" and the "why?".
This whole issue of simulated pain is part of the premise for Roko's Basilisk [0].