The biggest possibility is that the athletes are following the examples set by others who won before them OR
Biting the medal (metal) has become more of a tradition.
We have seen the athletes receiving their medals post which they all stand for the photo session. There is one major point that arouses our curiosity. Almost all athletes bite their medals once for sure. As a child we use to think that these medals are probably made of something sweet or to be more precise they looked like they were chocolates handed over to the athletes which signified that a winning person deserved something sweet.
But why do athletes bite their medals? Is it merely for the sake of biting in happiness or is there something else behind it? It is also said that the photographers are obsessed with this metal (medal) eating pose. They feel that the winning smile looks more complete while biting the medal. It is also said that biting on the medal especially Gold medal is to check that the medal is really made of gold. As per a book that proivdes more details of Olympic Games, human teeth are harder than gold. A quick gnaw to real gold would actually leave an indentation.
It is also to be noted that Olympic gold medals are have actually only 1.34 percent gold in them and the rest is sterling silver. The last solid medals were given at 1912 Stockholm Summer Olympics. When World War I began and gold was rarely available, alloys were added to reduce the amount of gold.
What seems more appropriate now is that biting the medal actually became a tradition and the players are merely following what the players in the olden times did. They have also seen photographs of their own family members biting the medal and generation after generation, the same thing is being followed and becoming more like a followed ritual.
Since the medals are not pure gold now, the winners only pose for the photograph and the actual biting ritual is long gone.