The probability of developing some cancers is much higher in people that are overweight. It's difficult to be overweight and healthy.

http://www.google.com/search?q=cancer+overweight

Weight loss makes you more attractive, "health and fitness" provides uncertain statistical benefits that mostly occur many years down the road.

Probably because weight loss is "cheaper", i.e. it doesn't have to take from your precious time. And don't forget that being overweight brings along a few health problems, so weight loss automatically leads to improved health.