system
Given the following post, respond as if you're commenting on Reddit:
human
[Witcher] Why aren't the Kings forcing the Witchers/Sorceresses to share their secrets for a long life?

Basically force them to make a surefire method of immortality or at least a method to prolong their lives by a few hundred years.

Lots of Chinese Emperors have died trying to perfect methods to extend their life and ironically dying as a result of said method.

None of these kings in the witcher seem to be interested in their idea of immortality.
gpt
1. Witchers are seen as sub-human. People view them in the same class as the monsters they hunt. Class-obsessed royalty would never consider debasing themselves to the level of witcher.

2. The nobility does exploit the witchers and sorceresses as much as possible. Those being exploited have their own agenda however. 

3. Very few potential witchers survive the mutation process. And it transforms you into a "monster." Which, as stated previously, no royalty would want due to loss of status (see: Adda the striga; also Ciri).