Because it turns out, and trust me on this one, that having a shit life causes depression. In particular: a shit life that the subject feels no hope of getting out of.
I suffered really severe, debilitating depression for years. I tried all the usual crap that know-nothings recommend, doing exercise and the like, and working hard to try and do better at life, but none of it budged the needle. Beyond that I was told I of course need to drug myself to feel better (SSRIs).
The depression was permanently solved, not by any of that, but just ditching my old life in full, and doing something else. Once your life stops being shit, you stop feeling depressed.
All the "chemical imbalance" talk is to avoid having to think about this, and instead reduces the problem in the same way a heroin addict does.
The main complexity in fixing it is that the subject often doesn't know their life is shit and just keeps trying to do that life better, or that they fear leaving that life will make things worse.
My non-psychologist tip for anyone suffering from severe depression like I was is simple: you have to make a very big, easy change to your life.
Big means not tweaking this or that, but completely changing everything. What's the most extreme change you could make, in your mind? Is it go and be a monk in Nepal for a couple of years? Then do that.
Easy means it can't be a struggle - if you're depressed you don't have the energy to do that.