Empathy is our ability to simulate reality from another perspective. When we empathize with someone, our brains are literally running an entire separate simulation, attempting to see from the eyes of the other person. When we deal with a fake person, inconsistencies in their behavior make it difficult for the brain to do this. Instead of a simple, natural perspective, the brain is picking up all these incongruent rules in the other person's behavior and it gets overloaded.