because it's not something that can be diagnosed
if you're sad, by god, you're sad
if you just wanna kill yourself, forget the doctor, if you're not depressed then you're not depressed
Depression has physiological factors in it; It is an actual mental disorder and can be properly diagnosed if doctors take time and effort with their patients, and patients search for more than just the "simplest" (Or what they think to be the simplest) route to go. Granted, the latter might be difficult for those who actually have it, but you get the idea.
Over diagnosis is what discredits issues like this and ADHD, which symptoms lies close to "typical" behaviors (Usually to just a higher extent). And that is where the problem lies: people are so quick to jump to conclusions. Patients go to the doctor to get medication, many times there are no ifs, ands, or buts. Doctors become vehicles for legal drug obtainment. This is an extreme case, but once more, you get the picture. The same can be applied to parents who are convinced there is something wrong with their child. And even professionals are perfectly willing to trust a test that may as well be in magazine sitting outside of their office. It's quite a shame, honestly, as it can only hurt most people involved. Those that actually have these are typically put into a general grouping of "Well, they
might need help, but who knows." and those that don't are taking medication they don't need.
And also, "Feeling sad" isn't depression and any doctor who prescribes medication for a normal human emotion probably isn't the most experienced or the most caring. :U