In Season of Storms (the last book, written in 2013 by Sapkowski), Geralt works for Mages of Rissberg Castle.
He meets the oldest (human) mage alive, Ortolan (this guy created the mandrake potion used by mages to stop aging, ironically he was very old when he found out and is trapped in the body of a senile old man for many centuries).
During the meeting Ortolan says something very interesting to Geralt:
[...] Cosimo Malaspina and is student Alzur, Alzur precisely who created the witchers. They imagined the mutation, whereby the creatures in your likeness have been created. Whereby you are, whereby you travel the world, ungrateful. You should hold them in esteem [...]
Alzur, his masters and their co-workers (mentionned in the book, but not especially named) are very powerful mages and created many creatures with genetic experiments, their ultimate creation are witchers.
With their abilities they earned a lot of money, and you can be almost certain they have enough money for building or purchasing Kaer Morhen for experiments on witchers and then leave them as home/stronghold/more-witcher-factory.
Another important point: experiment notes from Alzur and others are extremely rare. Mages of Rissberg (basically, Alzur and his group built and founded the Rissberg Castle facilities for their successors) have only a few and no details of the experience.
That's why a renegade mage writes the Monstruum books, and launches an assault on Kaer Morhen, partially to destroy the place and kill every witcher (except Vezemir): obtain experiment notes and the secrets of the witchers' abilities for himself.
The people who know where Kaer Morhen is located are very few. Except the witchers, some mages (Triss, Yennefer, maybe high-ranked Rissberg mages, members of mages council), Ciri, and maybe some guests (Jaskier-Dandelion for exemple, but I'm not sure, he just can't keep a secret).
And people are generally afraid of witchers.
They can try to kill one, if the situation is a big mess, but the cost in human lives is very high, witchers are super-humans made for killing creatures an army can't stop. And I don't think attacking a secret-hidden place somewhere in the mountains with an unknown number of witchers is a job you want.
Yes, it's happened once, but the mage who attacked Kaer Morhen pushed the resentment of people towards witchers for years (or decades?) before attacking.