The robbers sell their finds to traffickers in order to put food on their tables. Little is done to stop this. Too few people police the sites. When robbers are caught, they are not generally convicted or they receive a suspended sentence. Buying and selling artifacts is illegal, but the penal code refers only to grave robbers.
Traffickers appear to be well connected and thus avoid punishment.
Possession of artifacts is not a crime. Ironically, the U.S. is more aggressive in intercepting stolen artifacts, returning them to Peru when found. Draw a diagram of the market for artificats procured through grave robbing. Show the marginal private cost and benefit curves and the equilibrium price and quantity. Consider the news story and refer to the positions of the two curves in your diagram.