Because
they can't change their recommendations without admitting they were wrong, and governments generally
can't do that. Or, at least, the speed with which they can change their recommendations is sharply bounded by ensuring they never quite have to admit fault. Admitting you were wrong is basically begging to have your funding cut, your reputation trashed, your influence removed, your job cut. The closest thing a government can do to admitting being wrong is if they can say a
previous government was wrong, but now we're right. But for these administrative posts there's never a clear turnover for that to occur.
Public policy adds the additional problem that if they admit they were wrong about what was possibly the single clearest message the government has sent the US public in the last 50 years, "Fat is bad for you!", the public will very likely and entirely rationally start asking very pointy questions about how such volume could be combined with such error, and what else being communicated with such volume and confidence today may also be wrong.
The government has a ton of moral capital tied up in fat being bad for you, so it has to be bad for you. It has to be bad. It just has to.
As an individual, you may find it profitable to consider whether this is the sort of incentive structure that produces an authority you want to bet your life on. Considering whether there are similar issues where the government may have gotten stuck on a particular answer and now can not change its mind pretty much regardless of what facts come in left as an exercise for the reader.
Public policy adds the additional problem that if they admit they were wrong about what was possibly the single clearest message the government has sent the US public in the last 50 years, "Fat is bad for you!", the public will very likely and entirely rationally start asking very pointy questions about how such volume could be combined with such error, and what else being communicated with such volume and confidence today may also be wrong.
The government has a ton of moral capital tied up in fat being bad for you, so it has to be bad for you. It has to be bad. It just has to.
As an individual, you may find it profitable to consider whether this is the sort of incentive structure that produces an authority you want to bet your life on. Considering whether there are similar issues where the government may have gotten stuck on a particular answer and now can not change its mind pretty much regardless of what facts come in left as an exercise for the reader.