To respond I need to separate mental illness into three categories.
The first category contains illnesses like schizophrenia, with schizophrenia you can induce symptoms of the illness in healthy people by giving them medicine used to treat parkinsons. We can point to specific structures and chemical reactions in the brain as a cause for the illness, it may not be perfectly understood but we have a good understanding of what is going on.
The second contains things like personality disorders. It's not clear these are actually related to physical defects, at least in the same way the first category is. For example people who meet diagnostic criteria for antisocial personality disorder often report no symptoms and are successful in day to day life, some even thrive. Borderline personality types are often distraught over the state of interpersonal reletionships, that is the primary source of their suffering, however they behave in ways that destroy those reletionships. These behaviors aren't accidental, they involve sophisticated thinking, borderlines spin complex webs of lies to justify and hide these behaviors.
The third are things that are normal human suffering. Most people feel anxiety for a big date or a job interview. Most people feel sad when they lose a loved one. Negative emotions are a part of life. It's hard to draw a line on what is normal but a nonzero number of people are medicated for having normal human emotions.
It may not be helpful for the second and third category to be enabled in the way they have been.
For the second group, as the root of their strife is generally interpersonal, and only changing day to day behavior, over a lengthy period of time will have any effect in alevating the true symptoms of the illness. We can pretend the problem "isn't their fault" but essentially all their issues can be tied directly to their own behavior, it may not be their fault but it's certainly their problem.
For the third it isn't clear that they need to be treated at all, if you are too sad to go to work because your son died a month ago, nothing is wrong with you. Perhaps people are sad in society in general because the society in general sucks, medicating them in mass lowers the collective motivation to fix the society, what's the end game there?