Mental illnesses like cancer are defined by there effects not there underlying cause. As such they tend to treat a wide range of physical causes under the same general umbrella. And drugs or therapies which work wonders for some people often have little benifit for others. However, there are well studdied physical mechanisms for things like specific types of addiction which do present a physical mechanism.

A great example of this is Heroin addiction and alcohol addiction are vary different biochemically. There is even a wide range of Alcholhol addictions, but you will see them treated side by side. The simple truth is we can't really understand why someone has issues but we do have treatments that help people lead productive lives and that's enough to be useful. Just as your dentist does not need to know how pain killers work to use them effectively.