That is assuming, of course, that silver is or ever was an effective weapon against werewolves. The evidence for this is surprisingly scanty. Medieval Danish people believed that the most effective treatment for lycanthropy was to give the offending werewolf a stern talking-to. Other medieval cultures emphasized the need to bully and victimize the suspected werewolf by forcing him to exercise vigorously until he stopped turning into a wolf all the time.
This could possibly explain both the prevalence of bullying behavior and the popularity of exercise gyms in our own culture. Not only would the genes for bullying others have been selected by Darwinian forces (because the ability and willingness to bully others would have given our ancestors a distinct advantage in ridding their own communities of werewolves) but the genes for fanatically exercising all the time would have been an evolutionary advantage as well (as it would have showed any potential mates that the fitness fanatic had his werewolf problem under control). Another popular remedy was to pray constantly to St Hubert, but the genes for this one seem not to have been passed down, so it must not have been very effective. Imagine the surprise of a typical devotee of St Hubert as a werewolf relentlessly stalked and killed him despite his pious affirmations. That's why you don't see churches to St Hubert on every street corner.
Not until the 19th century did the silver bullet hypothesis gain any currency, and here there is another explanation. You can certainly kill an ordinary wolf by shooting a bullet at it, whether that bullet is made of lead or silver. If the same thing is true of werewolves, the silver bullet solution would seem to be effective, even though a lead bullet would actually have worked just as well. This fails to explain, however, why lead jewelry and silver jewelry are not equally popular. Does silver offer some other Darwinian advantage not shared by lead?
Or is it just kind of silly to think you can figure out every aspect of our modern psychology by speculating on what our remote ancestors might or might not have done?