Probably wont be taken well, but IMO depression is an illness in the spiritual sense. Depression is described as being sad, alone and without hope. That's quite literally what spirituality is for.

I think it's probably impossible to treat with drugs because what people really need are friends, hope and a good outlook on life (which often comes from hope). I'm sure drugs can improve people with imbalances and I'm also sure it's not just one thing that causes depression. That said, it seems like it can often be solved with more traditional means -- finding community and purpose.