A big part of this is health, the pharmaceutical companies having been rounded up into cartels by the FDA, the destruction of our farmland, poisoning of our animals and produce, and the overall assault on small business in both food and medicine.
People don't intuitively connect their mood with their diet. They think of the mind and body as separate, even though your nervous system stretches throughout your entire body, and even though a gut-brain connection has been established for a long time.
Sleep is also a big player that is not intuitively connected to mental health. Blue-light everywhere, high stress, little to no cardiovascular exercise, caffeine and sugar addiction means that our sleep quality is horrible. This causes countless health and psychological problems. Chronic bad sleep means chronic illness.
These aren't the only factors, but "chemical imbalance" is a marketing term that is meant to convince you that your problems are hopeless, and that only drugs can give solace. Similar to how P90X came up with "muscle confusion". There's nothing scientific about it, it's meant to mislead and convince.
If people started eating healthy food, and sleeping right, I believe that the majority of depression in america would be wiped out.
A depressed person doesn't want to hear that. I understand the difficulty, but it's still the truth.