The trouble is we don't really know the root cause of major depression. We know a little bit - e.g. it's partly genetic - but we usually can't 'resolve the root cause'. Treating the symptoms is not pointless.
"The trouble is we don't really know the root cause of major depression."
That's not really true. There are several different purely biological things that have been identified as root causes of depression: chronic illnesses, inflammation, nutritional imbalances, alcohol abuse, etc. These alone probably account for more than half of all cases of depression. (IIRC just inflammation is thought to account for 30% of cases.)
Similarly, there are many more lifestyle choices that clearly contribute: stressful work/family situation, lack of exercise, poor diet, poor social life, chronic sleep deprivation, etc.
Certainly there are individuals who are doing everything right and still end up suffering from a bout of depression that lasts more than two years even with therapy. However, these are fairly rare, as in almost certainly less than 5%.
As for treating the symptoms, it's not pointless but it can be extremely damaging. It can lead to more more severe mental illness (bipolar, schizophrenia), chronic depression, psychosis, suicide, violent behavior, sexual disfunction, etc.
There definitely are times when drugs are appropriate. But telling someone who has had suicidal ideations to look into drugs as a first line treatment is extremely irresponsible. It's like telling a 13 year old kid with a pimple to look into accutane.
