I suspect (wildly guess, to be honest) that, like "cancer", "depression" (or even "Melancholic Major Depressive Disorder") is really a name we give to a collection of related diseases with similar symptoms. Teasing apart the various forms will likely improve treatment (and improve quality of studies).
Yeah I have known many depressed people and been depressed myself. It seems much more likely to be a symptom than a singular condition, considering how differently people seem to experience it (some with it channel it into being successful, others can’t even get out of bed) its comorbidities, etc paired with how wildly many semi-successful treatments there are (SSRIs, CBT, diet, supplements, exercising/losing weight)
