Because the medicalization of depression is a convenient way to avoid thinking about the fact that it is a prominent feature of the status quo, not some aberration. Pollution correlates with industrial manufacturing and warehouse work environments and crushing poverty that forces people to live in a manner consistent with economic efficiency rather than with human happiness and well-being. Long work hours and little money contribute to social isolation. Debt traps put people in situations in which they could not possibly ever claw their way out.
https://www.ncbi.nlm.nih.gov/pubmed/24121465
> A meta-analysis of pooled odds ratios showed a significant relationship between debt and mental disorder (OR=3.24), depression (OR=2.77), suicide completion (OR=7.9), suicide completion or attempt (OR=5.76), problem drinking (OR=2.68), drug dependence (OR=8.57), neurotic disorder (OR=3.21) and psychotic disorders (OR=4.03).
When are we going to start designing our economies around creating a healthy and enjoyable environment for humans to actually live in, and not arranging human lives around squeezing the greatest amount of economic efficiency out of them as possible? The way we make ourselves live is tantamount to animal abuse.