The author mentions how there can be a sense of few "success stories" in the psychiatric world. There have also been some articles lately discussing how therapy may not be as effective as research leads us to believe, which has some people questioning therapy as a whole.
But what it really comes down to is that psychological conditions are not really just conditions of the brain (or rather, they very rarely are only that). Mental health conditions are often related to complex social histories, difficult and stressful environments, and unhelpful policies and stigma. To treat a patient, we need to fix their whole social position, their relationships, and not just how they perceive things. We can do a lot to help with just the psychological piece, but it certainly won't solve the problem.
And when I say there are stressful environments, complex social histories, and so forth, those are not uncommon. Loss of a job can lead to a stressful financial situation, which can strain relationships and push a person to live or work in areas that are less safe. There are any number of other possibilities, and nearly all of us have probably passed through times in our lives that could lead to mental health challenges. These situations can happen to anyone, and that's why mental health conditions are seen in all groups.