Why are there so many mental health problems in academia?
I think that part of the issue of why it seems there are so many people with mental-health issues is systematic. I think that academic environments are beginning to place much more emphasis on the mental well-being of their community members (students, faculty, and staff), so diagnoses of mental-health issues have increased compared to the past. Moreover, because universities tend to be more "accepting" of issues than most other environments, it's easier for people to mention such issues. Also, because of the nature of the people in academic environments, they're much more likely to recognize that they're experiencing mental health issues, rather than "passing the buck" or pinning the blame on someone or something else.

One other question that you haven't raised is if there's a greater correlation between increased academic training and propensity to experience mental-health issues; I'm not sure about the answer to that, and whether that skews the stats.

But in general, I think that what you're seeing is more likely something closer to the actual level of mental health problems in the general society in academic environments, and an underreporting of it outside of academia (where it is still heavily stigmatized).