Generally, for diabetes to use your example, there is an improvement if it is actually diabetes. They may eventually die from it, but their longevity and their relief from symptoms is increased. In the case of many of the drugs prescribed for mental illness, the results are very inconsistent, and on average detrimental.
Consider hypertension. The antihypertensive medicine may control the BP but it doesn't cure it. The patient goes on to have increased rate of stroke, coronary artery disease, kidney disease, etc, even with the meds.
My point wasn't simply that those taking the psychiatric drugs aren't cured, but that overall they are worse. If a doctor were to give you a medicine to control your blood pressure, but which causes immediate cessation of your heart...that's not really a help, is it?
We do everything possible to help the body cope with illnesses but I get the sense that some folks (possibly sometimes for conservative religious reasons)don't believe in mental illnesses. Somehow that is different -- not really part of the "body". My point is that the brain is as complicated and fallible as any other organ in the body.
There certainly are people who do not seem to believe in mental illnesses. I am not one of those.
The problem starts with the brain being so little understood, and the sphere of what can be controlled so broad that diagnoses are extremely subjective. It is difficult to even describe mental illnesses in a definitive manner, much less to determine what causes most of the problems.
Further, degree programs tend to self-select people that are mentally unhealthy themselves, often maniacal and suicidal to levels well beyond the population at large. Further, in my experience, there is an overwhelming tendency for the people who choose to study psychology not have any comprehension of the limits of the theories they study and to treat everything as a nail.
Repeated studies have shown that the most common result of psychotherapy is to become addicted to psychotherapy. They feel better about going to therapy, but don't really improve the underlying problems, and often use the therapy to justify continued ill behavior.
This is not by any means to say that there are no improvements, and no pharmaceutical aids, and no fundamental malformations. My point is that the brain is so complicated and so little understood that, beyond the benefits of a paid friend, the fields of psychology and even psychiatry are often little more than alchemy. This is a field where there are very few touchstones, and where abusing patients with pet theories is encouraged.
To go back to an earier example of yours, we can definitively measure the bloodpressure, but definitvely measuring "crazy" is not a straightforward process, and most of the time psychologists and psychiatrists don't even attempt to do so.