Yes, that certainly could be the case. Unfortunately there doesn't seem to be much (if any) understanding of what is different about those illnesses, and in par

Yes, that certainly could be the case. Unfortunately there doesn't seem to be much (if any) understanding of what is different about those illnesses, and in particular no practical test that can distinguish between them to help identify the proper treatment. By contrast, it is now very well recognized that there is no single disease called "cancer". There are many subtypes, not just based on tissue (e.g., breast cancer vs. lung cancer) but also on the molecular etiology (i.e., which mutations are present in the tumor and therefore which pathways of oncogenesis are involved). Because of this, it is in some cases possible to determine based on DNA sequencing which is the appropriate drug to use to treat a particular cancer. Depression is not anywhere near as well understood.