They aren't. The eyes are shut, and it has been shown in some computer enhancements that there are coins on the eyes. The coins have even been identified as a Pontius Pilate Lepton with a misspelling of "TIBEPIOY CAICOPOC" which would normally have appeared on a roman struck coin as "TIBEPIOY KAICOPOC" and the coin a denarius. . . on one of the coins and on the other a spiral coil. At the time the misspelled coin was identified, no such coin was known and it was claimed the misspelling was proof it was a fake.
However, several years later a couple of the Pontius Pilate Leptons with the misspelling were uncovered in a archaeological dig, and another was found in the British Museum collection proving the misspelled coin was authentic. There have now been six Pontius Pilate Leptons discovered in various digs and have been attributed to the 16th year of his Tiberius reign. . . which would have been about 30-31AD.
Pontius Pilate Lepton on the right eye of the Shroud of Turin
Another researcher claims it is a Pontius Pilate Lepton on both eyes, but the spelling is correct:
Never the less, the eyes are closed, and under computer enhancement of the image, it can be seen that there are coins there.