[quote=@rivaan]
[@VKAllen] I assume it's because well magic, but then again silver isn't inheritently better for swords in fact witcher silver swords are simply specially coated steel ones most of the time from what I remember. Steel simply cuts where I believe silver has additional effects on monsters.
[/quote]

In accordance to the original books, the witcher silver swords are in fact pure silver. Also, when playing as a witcher in the game, it specifically penalises you for using a steel sword against a monster enemy by reducing the amount of damage you deal to the enemy (except this is an exception in terms of several characters in Witcher 3, for brevity sake I suspect?). I conclude from this as the fact that silver weapons in the Witcher universe contains the same properties as that of many other fantasy worlds where silver is a purifying and holy metal that is able to penetrate an abomination.

Edit:

I am not dismissing the fact that silver-coated steel weaponry weren't used, in fact Geralt himself in the books would instruct other combatants to do so when there are monster infestations.