As for the not-understanding-it part, how is it hard to understand? It's effective marketing. The industry doesn't make money by people getting better, it makes money by getting (their) drugs into them. Humans are a vehicle for the delivery of their product. (So now are pets. The opening of the pet market for antidepressants is so classic a move that it will surely be studied in business schools.)
The vegetable oil industry did the same thing, ingeniously, against dairy fat. Remember those decades of propaganda about how butter and cream mean cholestorol which means heart attacks which mean death? It was an industry contrivance. The original studies it was based on were shamelessly cherry-picked. The campaign leveraged the widespread public fear that derived from the heart-attack epidemic of the 1950s and 60s. (Edit: I learned this from a CBC Ideas episode a few years ago.)
Just Google treatment for depression.
With depression, there are several things that can cause it: several known nutritional deficits, prescription medications, chronic illness, lack of social interaction, lack of exercise, poor diet, stress, etc. And similarly, the vast majority of people respond better to treatments that don't involve drugs.
There almost certainly are some people who do have underlying problems in their brain, but even still the depression is a symptom of whatever this disease is; calling depression itself a disease not only prevents the people without biological problems from getting care (the vast majority), but it also prevents scientists from looking for the root cause of the biological problems in the people who do have them.
Also, I don't know what root cause you're looking for beyond correcting the imbalances in serotonin, norepinephrine and dopamine.
I'm not sure if fixing the modern world's tendency towards crowding, emotional isolation, biologically toxic environments and stress is really in the jurisdiction of my physician.
Disease seems to be used to cover loads of things that IMHO aren't really diseases these days - eg alcoholism, obesity.
I think maybe it's to excuse the person of any guilt - "I can't help it! I have a disease"
But the claim is almost always "Depression is a disease caused by a biological problem in the brain." And the solution is usually just drugs.
I've never done any research into this area so I have no what the literature says.
And yet we have all these people committing suicide because they can't get the help they need, and we have all these people who freak out whenever you say that depression isn't a disease. I don't understand it. I've literally never seen anything where the academic research was so clear and unambiguous, and yet completely rejected by virtually the entire general public.