That is because they are PAINTED. Even modern attempts to duplicate the image using whatever means are MORE distinct than the image on the Shroud. Hair obeys gravity, and is generally uneffected by rigor mortis. HAving hair much like that, I can attest to that fact.
Now soak your hair (no I am not telling you to go "soak your head") in a mixture of heavy sweat and blood and allow it to dry while hanging your head forward so the hair falls forward and down. That puts a mighty strong stiffener in the hair. Then put a chin strap on, under your chin and beard, up and around your face in front of your face with the hair pulled forward and THEN lie down and see what you get. This has been demonstrated and the image on the Shroud matches the demonstrations.