Why is it that when someone has bariatric surgery, they first have to go through a gauntlet of all kinds of specialists?

someone i know just had bariatric surgery and he first had to go to a nutritionist twice, a psychologist, do a sleep apnea test, in addition to regular doctors. i could see having to go to a regular doctor, which is standard for any type of surgery, to ensure that the patient does not die during the operation or because of the operation, but why for the bariatric surgery do they need the sleep apnea, the nutritionist, the psychologist?