According to the recent times, it’s mainly because the photographers incessantly ask the medalists to pose as such on the podium for the photo shoot sessions, but in a way these photographers have helped preserve a long tradition, which would have been lost otherwise.
The tradition probably dates back to ancient history, where people had the practice of biting gold, to test if it was real gold or not. As you may have known, gold is a very soft metal and if it’s really pure (24 karats), you can probably see your teeth marks after biting it. (Don’t try this at home, you may end up losing your teeth. The gold you buy isn’t actually pure, it’s mixed with other metals to provide it strength)
Apart from the checking the purity of gold, this practice also allowed the people to check if the gold object was just gold plated or actual gold. If the object was just gold plated, with some metal like lead or copper as the base, then the gold plating could be easily scrapped off with teeth. So one didn’t have to bite too hard to discover the reality, given that the gold coins weren’t thick, so the plating tended to be very thin.
The Olympic gold medals aren’t actually made out of solid gold (they were till 1912), but they do have 24k gold plating. They are rather made mostly out of sterling silver. But if you were to try to bite a gold medal, you could actually leave a dent. This is because of the fact that silver is softer than tooth enamel, though harder than gold.
On checking the Mohs mineral hardness scale, we find out that tooth enamel is rated at a 5, while gold at 2.5 and silver at around 2.7. Tooth enamel is stronger than most common metals like the cooper, which the “Bronze” Olympic medals are made up of, so you could leave bite marks there as well.
This hardness scale created by Friedrich Mohs in 1812 calculates ratings by scratching one material against another. The one which leaves a scratch on other is rated higher. If both of them leave a scratch on each other, they are rated at the same level.
Well, it is obvious that none of the Olympian medalists aren’t actually checking the purity of gold (even if they are, we don’t know, but we definitely know that they do not want to have bite marks on their hard earned medals). The credit for carrying on this tradition can be attributed to the stubborn photographers whose incessant nagging forces the athletes to give in and bite off their medals. According to the photographers, it is more of a “playful” shot over the athletes just simply holding it next their faces.