Why would a health insurance company not want to pay for something that will save them money in the long run, even if they have to pay in the short run (for example weight loss surgery)?

A friend of mine is interested in having weight loss surgery. I am also interested in this surgery and was discussing it with her (discussing researching it, the next step, the doctor i am considering, etc). She has a multitude of problems that would surely benefit from weight loss: diabetes, high blood pressure, heart problem. She told me her insurance company would not pay for it.

Why would an insurance company not want to pay for something that may cost them a few thousand dollars right now, but ultimately will save them the cost of the meds, the cost of possible joint replacement surgeries down the line, the cost of hospitalizations that the other illnesses might require?