Because it is still medicine. You may disagree with cosmetic plastic surgery, but not everything they do is just cosmetic. I know someone who had their chin rebuilt after cancer... by a plastic surgeon. I read about another person who had a hole in their head filled in... by a plastic surgeon. Cancer patients receive breast implants after mastectomies... by a plastic surgeon. Some people need help losing weight, which can be achieved by using a plastic surgeon.
I believe all of these are ethical uses of medicine and should be covered by insurance.
Other uses, like breast enlargements and cosmetic surgeries, are not needed and most insurance doesn't cover it. If these people really want it, then they have to pay up front. The reason why I think this is still needed is that people would still get the surgeries if there were not plastic surgeons. They would just go to backyard clinics and get them done, with higher fatalities and infection rates. At least with plastic surgeons they get medical training and have the proper facilities to do these surgeries.
