The claims you are making are stronger than the claims made by that paper (
http://www.sciencedirect.com/science/article/pii/S0149763416...). First, the paper does not claim that cortisol
production is related to psychosis. Instead, it measures the degree to which cortisol levels increase immediately upon awakening (cortisol awakening response or CAR), and specifically notes that CAR necessarily related to overall cortisol levels, but suggests that it may be related to hypothalamic-pituitary-adrenal axis function. It finds that CAR is blunted in patients with schizophrenia and first episode psychosis.
However, it is not clear whether the changes in CAR are causal or symptomatic. For example, psychosis might lead to a disrupted wake-sleep cycle, which could then lead to blunted CAR. To determine whether there is a causal effect, you'd need to perform a longitudinal study, measuring CAR in patients who are at first normal but some of whom later go on to develop a psychosis, and show that the CAR differences predate the development of psychotic symptoms, instead of developing later. The paper notes that such longitudinal studies are "clearly needed."
Mental illness is complicated. While it can clearly be explained scientifically, if the answer were easy, we would have found it by now. With that said, existing treatments have been shown to improve outcomes in double-blind placebo-controlled studies, even if we can't fully explain why, and that is why they are used. There are certainly problems with these treatments: They don't work for many patients and they often have undesirable side effects. But overall, we know that they help people, which is why we use them.
There are undoubtedly better treatments out there, but either they haven't been discovered yet or they haven't been proven to help. There is no nefarious motive behind sticking to the status quo. Researchers know that, if they were to publish a paper showing spectacular results from a treatment with few side effects, it would be great for the people they're trying to help, and also for them and their career. Instead, people stick to the status quo because, historically, alternative treatments have not been as effective.