"The Others have a few known weaknesses that are recorded in ancient texts. One is obsidian, otherwise called dragonglass or "frozen fire". When Samwell Tarly accidentally stabs an Other with an obsidian dagger, its flesh and bones melt away leaving only an icy puddle. Ancient texts also record a weakness to "dragonsteel", which several have taken to be Valyrian steel. Mance Rayder expresses belief that magic wards in the Wall prevent the Others from crossing into the Seven Kingdoms.
After listening to Tormund’s words Jon Snow remembers all the things that Sam had told him, the things he’d found in his old books. He thinks to himself that Longclaw had been forged in the fires of old Valyria, forged in dragonflame and set with spells. Dragonsteel, Sam had called it. Stronger than any common steel, lighter, harder, sharper … But he thinks that words in a book were one thing. He knows that the true test comes in battle." -From awoiaf
So there. I think it's because you only watch the show and don't read the books.
