That's a great question.
I agree with you, the resource situation means that there is already some difficulty in accessing mental health care. The difficulty that stands out is making sure an independent assessment is obtained.
Since the therapist may have been meeting with a patient for several years, the therapeutic relationship could be tinged with certain elements. Having an independent third party confirm the opinion of the treating physician appears to be a source of safety to ensure that one is not moving a little too quickly or missing something important.