For once, people work too much. It exhausts the body and we feel tired.
For two, there’s the meaninglessness of life. It’s difficult to stay motivated when nothing makes sense/there is no future.
Thirdly, positive sexual experiences strongly cure depression. Since the dating market is largely fucked (no pun intended), well that option doesn’t exist to large parts of the population.
Fourtly we’re socialized to hide depression. As everybody knows, the first step to solve a problem is to recognize it exists. Stigmatization of depression has held back effective treatment for way too long.