Furthermore, "depression" as discussed in every day conversation can be a normal mood that is part of chronic unhappiness, or that occurs in response to grief at someone's death or due to any other loss or misfortune.
Furthermore, someone in a major depressive disorder episode reacts completely differently to life's every day ups and downs than they do when they are not in the middle of such an episode. It's almost Jeckyl and Hyde territory.
And people with suicidal ideation, comorbid (other, co-occurring) conditions, and significant personality pathology are excluded from studies. Those "exclusions" eliminate the vast major of subjects that have any of the psychiatric disorders in which depression is a symptom.
Garbage in, garbage out.