I think the problem is that in computer animation skin is allowed to stretch and compress in ways which is unnatural. Real skin is quite inelastic. It wrinkles

I think the problem is that in computer animation skin is allowed to stretch and compress in ways which is unnatural. Real skin is quite inelastic. It wrinkles and slides which creates the appereance of elasticity. Also, when real materials are stretched or compressed, they tend to form wrinkles in particular ways. I think that human eye can tell these very small discrepancies to guess that this material is not human skin (or any physical material).