I think, without sounding too disconcerting. Some people are going through stages which are normal. It may be hard for some to distinguish between mental health issues and teenage angst.
A lot of these issues are associated with emotions, so some may feel as though depression is just sadness and anxiety is just shyness, which they have "felt before", when it is not.
It's hard for some people to understand. To a person who doesn't have these problems, it can be difficult for them to understand why I starve at times, because I don't want to go to the shop or why I just randomly start thinking about why I dislike life and shut myself off for some weeks. They see the solution as easy, "Just go to the shop" and "just go to the doctor", but when they don't experience these feelings, they cannot understand how these tasks can be difficult.
Desensitization could play a part, I've heard people say "every man and his dog has anxiety and depression nowadays". So this repeated exposure to it desensitizes people.
Also, kids are cruel.