I don’t think there’s any great mystery to this. We want to breed with the mate most likely to help us produce viable offspring. Therefore we’re sensitive to indications of health and good genes. Symmetry, smoothness of muscle movements, quality of skin & hair, indications of good blood flow, even things like regular breathing are all indications. When we see a simulation that appears not quite human it’s noticable for all those details. That detection of unhealthy can easily detect death of course. There may also have been an instinct to avoid the sick, but social pressures override that sometimes.
The internet theory that we wanted to detect imposter humans is silly. Early hominids interbred a lot. It’s not “human” that we’re sensitive too. Just health.