Psychiatry is... a super new field outside of holistic or traditional medicine. The aim is, i think, noble and interesting: to try to marry understanding of behavioral health and physiological health / mind & brain. That said, psychiatry is probably still establishing itself in Western medicine, in the realm of molecular understanding of disease and health, and it has a long way to go as far as the brain / neuroscience is concerned.
Borderline personality disorder, bi- and unipolar depression, and other mood disorders don't have definitions that lend themselves to treatment or cure by chemistry. That is, the most robust, scientifically verifiable understanding / definition of such 'illnesses' is limited to behavioral terms, not physiological or neurological ones.
Let's put it this way, from someone who has education/experience in pharmacology and drug design, using cancer as an example:
Drugs (chemical interventions) work on the scale of cells and molecules in our bodies, and so the understanding of how they work and their development in the present lies at that level. Every time anyone takes a drug, they're nearly carrying out an experiment against their state without that particular drug.
For cancers, there is a scientifically validated definition for what cancer is as a disease (down to the cellular / molecular level), and a whole explanation for the progression of that disease in the body. Drugs for cancer that are being developed / discovered are tested by exposing tumor cells (whether they're grown as cultures, outside of a body, or in test animals) to the experimental drug and checking for changes in the size of the tumor.
For psychiatric illnesses (e.g. BPD, bi- and unipolar depression, anxiety) there is not a scientifically validated definition for what those 'illnesses' are on the cellular / molecular level. We have drugs like SSRIs that are prescribed for mood disorders, and some statistics that suggest they can reduce symptoms associated with such mood disorders as they are defined, often in combination with psychotherapy (another type of medicine / experimental variable unto itself). We say, as with SSRIs (selective serotonin reuptake inhibitors), that such drugs may or may not be particular powerful accelerators or decelerators of some specific cellular activities in the brain, BUT no one actually has published proof for how perturbing such cellular activities is related to 'depression' or 'borderline personality disorder.' (an aside: this amounts to what might be interpreted as an extraordinary opportunity for $$$ in drug discovery, being that there exist multiple hypotheses for what depression even is, for example, so if you already have the means to invest, you can go for many shots on goal as possible in terms of projects whose goal is to develop a psychiatric medication. If the new drugs the safety tests in humans and show even just a hair of a positive difference over the current standard of treatment, you won't necessarily have a harder time persuading doctors to prescribe them than the next company with a drug for the same condition that works via a different mechanism)
Brain is not a synonym for mind.