Actually, the chemical imbalance theory is on shaky ground at best. It's convenient for marketing antidepressants to be taken for life, but that's about all it has going for it. Otherwise, why would side effects hit as soon as you start taking the drug, but the benefits lag by about 6 weeks?
There are plenty of reasons why that may be. The chemical imbalance is the best theory given the data available (yes people do research on that from time to time).
I'm not saying there are no benefits, just that correcting a simple chemical imbalance doesn't seem likely to be the mechanism.
As for fixing what is making people unhappy, just because a pill can correct it doesn't mean the origin of the problem is internal. Consider, an NSAID can help with the pain and swelling of a strained back. Does that mean the problem is an inherited imbalance in my back rather than my mistaken belief that I can move that fallen tree by myself?
Of course people can be unhappy from external causes, that is just natural. Depression is marked with differences in thought patterns and behavior that makes it much less likely to resolve by itself even if any external causes are resolved. While I realize that the side-effects can be severe for some TCA (tricyclic antidepressants) and ECT (electro-convulsive therapy) helped me when severely depressed. I still try to inform people that ECT is a safe and extremely effective treatment that helps in some cases where antidepressants have no effect.
Schizophrenia shows all signs of being controlled by genetics though there is some evidence that external conditions can hasten or delay onset. Susceptibility to depression may also be genetic, but there is significant evidence that it can be brought on or sent into remission through external influences only. For example, seasonal affective disorder.
I wouldn't be at all surprised if the problem is some combination of being always on, multitasking, poor sleep hygiene, and poor financial security. The constant IV drip of fear from the media and our "fearless" leaders isn't helpful either.
Plenty of people get depressed without external reasons. Postpartum depression can be extremely severe even if the child is wanted and the life situation is perfect. People that "have it all" can suddenly get depressed and kill themselves without anybody (including themselves) knowing why. People that have starting to get depressed can suddenly treat small things like someone not greeting them loudly enough (even it they were greeted with a smile etc.) probably meant that someone was insulted and/or irritated by the depressed person - had to explain once that I didn't despise a depressed fellow in exactly that situation.
Life is complicated. Make lemonade.