There are a few reasons for this.
1. It is not available in a widespread way in the public hospital system. So everything is provided by private practitioners and private hospitals. Health funds will cover a lot of the cost but not quite all the way there. For insured patients, the out of pocket cost is about four to five thousand dollars.
For uninsured patients the costs can come to about $18000.
Many patients will access their superannuation to finance their surgery.
2. Weight loss is not just about the operation. Success depends on the input of dietitians and psychologists. These all cost money
3. There is a longer post-operative followup period which is different to other forms of surgery.
4. Gastric sleeve and gastric bypass surgery can carry complications. These need extra work which the fees account for.
Bear in mind that the cost of surgery is recovered by food and medication savings within one year. Also improved quality of life, mobility and energy levels is priceless.