Psychiatric medicine seems to rely on a lot of trial and error, to see what works and what doesn't. The problem compounds as soon as multiple medications are used. Differences in physiology and the nature of the disorder, including factors such as incorrect or incomplete diagnosis seem to lead some unfortunates down the wrong paths, where the presumed illness is incorrectly treated. For example, some personality disorders, such as BPD, can't be effectively treated with medication at all, and must be tackled therapeutically instead, but are often misdiagnosed as something else, or, are sometimes only recognized as anxiety or depression, and while some SSRIs can help with that component, the bigger underlying diagnosis isn't recognized.
In short, treating mental illness can really only be done on an individual basis, and not every psychologist or psychiatrist is going to get it right, and more often than not, many mental illnesses are not treated correctly from day one. To some degree this is to be expected, but it also results in many people seeing nothing but confusion when looking at treatments which are in place. It takes time to get to know someone's mind, so it's no wonder that years can pass before even skilled doctors are able to make a positive impact on many mentally ill patients. It's also no wonder that many of these patients fall through the cracks in the mean-time. Not everyone can afford the kind of treatment required by some diereses, and the waiting game can be frustrating to even well adjusted people.
All I'm saying is that there are many problems which lead to GPs continuously prescribing a regimen of ineffectual drugs, without even thinking about issues such as poor education or practices, or the various evils, (imagined or otherwise), of pharmaceutical companies. To be clear, it's not the drugs that are the problem. The drugs actually do help in many cases. It's getting to that point where they do help that is the trick.