Why do the guidelines for what we're supposed to eat change so often?

As a human, you are inevitably a part of some sort of society, which comes along with a form of government that controls many things but among those things that are controlled the most essential to us is food. Our food isn't necessarily spoon fed to us in the portions they decide to give us at certain times; however, there are guidelines for what they believe we should eat. But are these guidelines really accurate ? Apparently not , well in the case of the U.S. that is. Our guidelines here in America change every five years , Just this year in fact we have received new guidelines. But why are these guidelines changing so often ? Its understood that things evolve with science but honestly how many things did you guys find inaccurate before ? Also they expect you to follow these guidelines to be healthy but they aren't even sure of them . I mean EVERY FIVE YEARS ?! if I were to live until i'm 100 i'd have changed my diet based on the guidelines 20 times ! Is that honestly a way to keep our bodies healthy, with constant changes in nutrition ? I mean other countries have kept their guidelines predominantly the same since it was created .. why are they able to understand and feed themselves properly and America cannot ? Aren't we the Superpower country ? Come on America we need to do better!