The key thing to understand about studies like this is that healthier people tend to be more productive and therefore are more likely to contribute more to society. It may cost more to treat the health problems of an 80 year old alzheimer patient than a 40 year old with lung cancer, but that 80 year old likely worked for an additional 25 years during which they paid taxes and helped to overall grow the economy.
So it is easy to measure the cost but the benefit is harder to measure (and this doesn't even count harder to measure benefits like happiness that healthier people tend to have).