Nders,
I see what you mean; it is not a popular trope of many RPGs.
While not being cannon, many myths, legends and folklore are ripe with these ideas; IIRC.
Nailing horseshoes above doorways to prevent spirits/fays/goblins is considered lucky.
Many cemeteries have iron fences... maybe to keep things in..
When other cultures' myths trickled into society, different items affected these "new" creatures. Silver for werewolves and even vampires.
Often, holy symbols and "certain books" I believe came later. If you find it hard to conquer a people, rewrite their pagan ways to match your own ideas..
Easter/Ishtar, Halloween/All Hallows Eve..
Even creatures hit by magic weapons seem to have a bit of the old left in for good measure.