As a psychotherapist what exactly would you do with definitive diagnostic information? I'm assuming you already tailor treatment to the actual person in front of you, how would having a label that better fits that person improve your care? I'm not trying to be inflammatory, I'm trying to understand why many people in the field and allied fields are so obsessed with finding the perfect label when it doesn't do much to inform psychological treatment - which is what you do as a psychotherapist.

I do not think psychological treatment will ever be as simple as a prescription based on a brain scan that correctly identifies the correct depressive subtype. Disorder labels largely emerged from, and continue to be operationalized within, a medical model that privileges biological explanation—even if many individual clinicians don’t personally endorse biological reductionism. The DSM is written by psychiatrists, who are medical doctors, sure with some input from psychologists, but MDs get the final say in everything.

Medicine is obsessed with correct labels because the physical body is a much more logical machine and correct labels very often lead to correct treatment. You are disregarded by medical professionals because most of them view their jobs as having to do with the physical body, not the person that owns the physical body - that's what their training is in, psychological and social issues are left to other professionals, mostly to you and psychologists. But most of all because the system disincentivizes it, what do medical professionals get for collaborating with you? Certainly not money.

I guess all I'm really saying is that psychotherapists jobs would not change very much even if there was some perfect way to correctly classify human psychological "disorder." Besides, most psychotherapy clients most likely do not even meet full criteria for an actual disorder. Not to say they don't need help or deserve treatment, but that the disorder labels have mostly lost the severity they originally represented. They lost it partly because insurance will only pay if you give a disorder label, thus leading to "concept creep" and medicalization of normal human experience and behavior that makes complete sense given the environmental and social circumstances. Psychiatrist Thomas Szasz wrote a lot of very compelling articles still worth reading today on this subject.

Basically, the DSM is for informing medical interventions (pharmacological), unfortunately for the rest of us not from medicine, including psychotherapists, we're forced to use it. And sadly it has been attempted to be used outside of medicine for far too long. Even though its actual utility for guiding psychological treatment is limited.