The argument proffered through is that it is necessary for their mental health. Which then begs the question, what about people who compulsively get plastic surgery? Which is exactly why the DSM carefully says that "gender nonconformity is not in itself a mental disorder. The critical element of gender dysphoria is the presence of clinically significant distress associated with the condition". Body dysmorphic disorder on the other hand is itself pathological, so continued surgeries do not rank as humane treatment.
Surgery is not the only possible answer to psychological distress, if it is one at all. Some people have body dysphoria and surgeons aren't required to weigh their distress about having an arm above everything else.