That's a fair point, but for some mental disorders, the brain is wired into a state of permanent anxiety, wherein it becomes extremely difficult to 'learn' that

That's a fair point, but for some mental disorders, the brain is wired into a state of permanent anxiety, wherein it becomes extremely difficult to 'learn' that normal situations do not deserve heightened stress responses. The most famous book on this subject is probably Bessel van der Kolk's book The Body Keeps the Score. For people who have been so traumatized that their ability to regulate emotions is nigh-totally kaput, medication can offer a window of relief that enables the kind of healing that sticks, even when the medication has worn off.