[rcxdude]

>but it would be impossible for an author to simulate a complete human being in their head.

Perhaps not (depending on your exact definition of 'complete'), but I'd argue as social creatures, humans have brains that are fairly well optimised for simulating other humans (and many authors report that their characters become quite firmly formed in their head, to the point of being able to converse with them and know directly what they would do in any given circumstance, even if that's inconvenient for the plot). In fact, we frequently pretend non-humans are humans because it makes it easier for us to simulate their behaviour.