It is easy to explain why antidepressants vary wildly even if all depression is caused by issues in brain chemistry: the constellation of symptoms we define as

It is easy to explain why antidepressants vary wildly even if all depression is caused by issues in brain chemistry: the constellation of symptoms we define as "depression" is actually several different illnesses that respond to different treatments. This is why when people start on antidepressants it is a process of trying different ones until they find one that works.