I'm going to go out on a limb and guess the prominence of depression is largely caused by a broken interface with the world at large. The traps created by modern society wherein you struggle for other people's benefit are better designed than ever before. What our social machine procedurally treats as progress is advances in subjugation. Depression may often be an artifact of "efficiency" with respect to extracting v…
Alternatively - or just, adjacently - with the internet (and big business, and widespread university education) we are culture-shocking vastly more people into a medium that can barely be understood from a detached intellectual and philosophical perspective, and is almost impossible to fathom from historical, traditional, ancestral or biological ways of understanding (the way humans operated for vast majority of our existence). We have little history to compare this new way of life to, and it's unsurprising if it's doomed to breed mental disease by its very nature. We have awakened far more people to an absurd reality, and "what is meaning?" is no longer just a question for a tiny group of overintellectual philosophers. Some of those philosophers say that one must walk through the valley of the shadow of death before we can find that answer for ourselves - but even they're gonna admit that a whole lot of people are gonna get stuck in that valley for a while if you just throw the masses in like we have. Hopefully that's just a temporary growing pain of our society as we gather our strength and do better but - time will tell!
(P.S. rich people are just as trapped in this malaise of meaninglessness. They have more agency, sure, but they're also still part of the systemic trap and unlikely to have much better answer on how to change anything than the common man. Only so long even a rich person can hide in hedonism before the depression catches up - or they just embrace the cold reality and adopt the "I got mine" attitude which said system is naturally inclined to. Depression may very well be due to our moralistic social being nature chafing with a cruel capitalist system which has won by almost every measure possible)