Nurse Ratched isn't really concerned with curing her patients of their mental illness. For one thing, a number of men on the ward are free to discharge themselves from the facility at any moment. So the nurse figures that it would be better to create an environment in which she maintains maximum control. Every single aspect of day-to-day life on the ward, from the soporific string melodies to the group therapy sessions, is designed to keep the men in line and Nurse Ratched firmly in charge.
Unfortunately, this means that at no point are any of the patients ever able to deal with their mental health issues in a meaningful way. They are being cynically manipulated by Nurse Ratched, as we see most clearly during the group therapy sessions. That's why the arrival of McMurphy is such a threat to the power she has over the men. In his own way, he does offer the other patients a cure, albeit one that you won't find in any textbook of psychiatry.