I think it isn't exactly rarity of GT adults that is the issue, but more the rarity of therapists who are GT. Highly capable adults have many options for careers, and the rewards of being a mental health professional are mostly intrinsic. But people have to be able to support themselves and their families, too. Not much (compared to other professions GT adults could enter) of either material or status reward accrues as a social worker or mental health counselor. There is a little more of both for doctoral-level psychologists, but probably not enough to significantly tip the distribution of GT individuals into the profession.
I don't think the therapist absolutely has to be smarter than the client for a successful therapeutic relationship to develop, but they probably do need to be within shouting distance.
