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I'm no expert, but from what I've heard, it's because natural surfaces like skin and fabric are all made up of multiple layers, each with their own degree of light absorbance and relflectivity. Only really, really swish, ultra-modern software can render all of those layers at once. Most just go for one or two and shove some flat reflection over that.
Again, no expert.
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Subsurface scattering.
Most graphics cards in the XXX nvidia series' can do it now. Still very expensive to render though.