But the people who're governing young taxpayers now will no longer be governing those taxpayers when they get older and will suffer from diseases caused by today's policies - policies which will raise the standard of living of the same taxpayers today. And a future government will simply blame past governments for the problem (rightfully, though not very usefully.)

"The government" (why the capital G?..) as an organization might have an incentive to protect taxpayers' health but individual people making up said government do not have the same incentive. I think they call it the agency problem.

(Some people go as far as advocating benevolent dictators with a hereditary right to rule; of course in reality few dictators are benevolent, and a hereditary right to rule prompts people to kill off ruling families. I conclude that setting up incentives such that everything works out smoothly is rather damned hard.)