it makes fighting monsters easier because they're weak to silver. But it does not make sense why no one else use it but which rooms. Especially because in the world literally around every corner there is a monster of some kind of an almost right next to settlements you would think at least some people would have silver swords

It makes fighting monsters easier, because many of the beings that appearing in the conjunction are weak or vulnerable to silver.

It doesn't really make sense that no one else uses this knowledge, other than Witchers - especially because in the world literally around every corner there is a monster of some kind, often almost next to settlements.

You would think at least some people would have silver swords.