Because it absolves them of any responsibility for their own or their children's behavior. They get to see themselves as good parents of good children who have a medical problem.
A friend of mine had a child who was diagnosed as Oppositional. My oldest son has Asperger Syndrome and was in therapy at the time (well before the diagnosis) because he was struggling socially and acting out at school. The friend explained to me all about how the therapist recommended therapy for her and her husband so they could learn how to deal with their oppositional child (who was at home). She expressed much surprise to find out that my husband and I were not in therapy over our son.
I very nicely did not explain that my child was in therapy because my CHILD (his thoughts, assumptions and reactions) was the problem.