Hard to say because we can not see the code. Subsurface scattering might be part of that equation.
I would just point out that human brains are extremely specialized in facial recognition. It has been postulated that the brain has a inbuilt defence mechanism to detect alien impostors/anomalous people. You are right in middle of what is known as uncanny valley nothing sort of near perfect seems to get it right, it seems
It is also worth noting that we do not really know what it is that is lacking that puts you in uncanny valley. So it seems to me we do not conclusively know what, if anything, is missing. So the entire thing might be because we are neglecting something important but as of yet unknown.