Philosophers talk about this in terms of` 'akrasia' or weakness of will, to explain how we know the good but do not always do the good. For example, students know that they will learn more from reading an entire play or novel than from skimming a summary or visiting a cheat site, but they may decide that other immediate pleasures (going out with friends) are more attractive than the long term education benefits of homework. Similarly, if you are confronted with a choice between a plate of tofu and steamed vegetables, which may result in a slightly lower chance of a heart attack in fifty years, and a bowl of ice cream, which tastes good right now, you may decide that the immediate pleasure of the ice cream is more appealing than a minor change in long term probabilities.
This is an interesting question. There are a few reasons why people do not listen to the experts when it comes to food.
First, some people do not know what the experts are saying about food. Not all people have the leisure time to learn these things, especially poor families that work hard to make a living. There are also a lot of immigrant families in America that may have language issues.
Second, to make healthy choices when it comes to food is not possible for many families. It takes a lot of money to buy vegetables. It is much cheaper to eat a bunch of hamburgers. This is a very unfortunate situation for many people and I believe that we need to do something about this.
Third, experts disagree and some people do not trust those who call themselves experts. In a postmodern world, it is hard to know who to trust.
Fourth, some people are simply too busy to make healthy decisions. Fast foods are fast; there is a convenience factor.
In conclusion, there are a myriad of reasons why people do not listen to the experts.