re: Why has mental health declined so much over the years?

I don’t think we evolved to have this much information about things that aren’t directly in our physical environment.

Same goes for people we interact with. Maybe the more local we are, the better.

Another thing is rampant commercialism/keeping up with the Joneses. “Comparison is the thief of joy” is a worn phrase now, but it’s a real thing.
Mimetic conflict, the kind of thing that from the dawn of history has led to real physical violence as people tried to take what others have, is still alive and well, but now it may be manifesting as violence we do to ourselves, internally.

Advertising is built on taking advantage of this inherent weakness, and advertising is now everywhere.