Because it's not as prevalent in other societies. The fixation of Americans, and especially younger Americans with mental health is not something I've (or clearly, GP) witnessed elsewhere.
I don't think the discussion here is due to a lack of empathy, rather it's curiosity of people looking into this society from the outside (which we're doing all the time because we live in an Americanized world, after all). It seems like the participants in this game of self diagnosis and mental health crusade are very self centered and not very fit to deal with life (which is a complicated matter, I admit to that).
This is not to dismiss the hardships of those people professionally diagnosed with mental conditions, obviously.