Suppose (without loss of too much generality) that I am anxious. There are two conceptualizations I could consider. First, the one given by the "ruling ontology":
I suffer because I am afflicted by an anxiety disorder. I must receive therapy & counseling in order to manage my experience.
In this view, I have an individual problem and need to solve it at the individual level. However, there is a distinct framing which suggests that there could be "social causation" for my anxiety:
I am forced to suffer because I am resident in a society which induces anxiety and have been brought to embrace anxiety as a way of life. We must receive changes in our social contract in order to improve my experience.
When we look at words this way, we can see that "mental illness", "personality disorder", and other common ways of characterizing reactions to our society are stigmatizing assumptions about individual medical issues which are intended to obfuscate any underlying social causes. Where you ask for "mental illness' causation", you've already assumed an individualist framing.
As Fisher points out, capitalism cultivates the former framing for its own benefit. Quoting from Capitalist Realism, after the portion in the question:
Considering mental illness an individual chemico-biological problem has enormous benefits for capitalism. First, it reinforces Capital's drive towards atomistic individualization (you are sick because of your brain chemistry). Second, it provides an enormously lucrative market in which multinational pharmaceutical companies can peddle their pharmaceuticals (we can cure you with our SSRIs).
This overarching concept is closely related to iatrogenesis.