Supposedly, the world in The Witcher universe used to be crawling with monsters so there would be no shortage of them, possibly with a semblance of civilisation too, monsters that could pass as men-at-arms to fight against.
That's actually one of the problems of the games, IMO. It's long since I read the books and only the two that were translated into German, but the major themes (IIRC) were that
1. Geralt wasn't so sure anymore if the monters were truly evil/if humans were worth saving.
2. If Geralt himself wasn't closer to "monster" than human.
3. That he has great difficulties finding monsters and getting contracts. He only encounters 2-3 monsters in the 2 books and often contemplates what place a witcher has in the world if he hunts all monsters to extinction.
In the games? There you wade through hordes of monsters and leave mountains of corpses behind you... So the games actually already played in that monster-infested age. Just sayin'.