Mostly because diagnosis' of mental illnesses largely come from what the patient tells the doctor and the doctors are very careful before making a diagnosis, unlike other illnesses that can be diagnosed with blood tests or x-rays. Mental illnesses are different for everyone. What may be anxiety to one person may be OCD to another. A lot of psychiatrists or psychologists would rather address symptoms and not worry about what to call it. This is important because a person may think they have depression but after some careful study there may instead be another cause for the way they feel not related to chemical imbalances (like vertigo). Hope this is helpful and that you know your question is not uncommon.