To ask why tempering is incurable is like asking why there isn't a cure for cancer.

It's not impossible, but neither is it going to be easy. Tempering, like a malignant tumour, changes an individual's mind/aether to such a severe extent that no one yet knows of a way to reverse its effects.

Case in point: A very significant exception has already been made, where one tempered individual was spared from summary execution, thanks in no small part to Alisaie's intervention. Even so, all parties involved are no closer to rehabilitating the individual, although there are signs that he hasn't lost all traces of his original mind.

It's worth reiterating that two things are needed to sustain a primal: Aether and worship. A primal is essentially an idea brought to life by intense belief. That remains the fundamental reason for executing the tempered: Keeping them alive only means it'll be easier to revive a defeated primal. It's too big a risk to take, in most cases.