I would completely agree with you, @Raz415, were it not for the myriad counterexamples present within the game itself.
Lest we forget, the Gondians are successful in fixing this exact problem, for a literal army. The only reason why this is not possible for Karlach is because it isn't present in the game, having been (incompletely) removed at the last minute.
As for your other examples, resurrection of a skeleton is a far, far different problem than fixing an Infernal machine - which we do quite easily twice and would be required only a third time. In Mayrina's case, the Hag didn't even intend to try fixing her problem, being specifically set on inducing pain in all of those who ask for her help.
Editing for completness' sake, the case of Ketheric is clear-cut grief-induced hubris. An extreme example of unhealthy coping mechanisms, if you will, and it completely makes sense within the bounds of his character and personality. He loved his wife to death, he was equally loved by her, and the loss of both her and his daughter broke him and caused him to look for solutions in all the wrong places.
These are, indeed, both part of the universe of D&D, but when taken as a whole, they are barely footnotes to the immensity that is the rest of it.
