Wow! At least you show, for the second time, you have never experienced depression and are only viewing this as through a window of a house you've never stepped in, but are assuming its history is exactly the same as every other house on the street!
And, in clear knowledge of this, you use reductive argument to underline your point; if water and food are drugs then we are what we eat?
Let's take your philosophy to a logical place of reasoning; I, along with many others, choose not to ADD man made substances into our already drug packed bodies. Many of us would rather 'self medicate' by releasing own natural chemicals, so we're strong enough to fight our issues ourselves or seek further help for that fight.
Simply painting with broad strokes may cover everything for you, but you miss the underlying details of the picture that's there.
So, yes, some may want the broad strokes, but some want to deal with the details when they're ready.
Don't judge them for that choice.
Oh, and by the way, comparing cancer/ leukaemia to mental illness is staggeringly stupid! Staggeringly!
The pathways may have similar traits, but you can learn to live with depression better than learning to live with cancer or leukaemia!