Most people don't understand why this is the case, but all good "regress to cause" hypnotists/hypnotherapists do! Being overweight almost always has something to do with a subconscious need for protection, or a fear or insecurity of some kind. These people people find security in food as it reminds them of the security they felt while being fed as a baby and then growing up. Once surgery removes this crutch, the fear/insecurity remains and another crutch is sought and found in drug use, alcohol use and cigarette smoking.
Some hypnotherapists will offer a hypnotic gastric band which is even more effective than the actual surgery, but probably come with the same risk unless the underlying subconscious insecurity has been dealt with. Nothing deals with subconscious insecurities faster than "regress to cause" hypnosis and a competent hypnotherapist.