It's simply that we only notice the rare instances where they did get it wrong. And in those instances, we have to ask why. Sometimes the scientists were in fact right, but human error occurred in applying that knowledge. Sometimes it was that the knowledge available at that time was insufficient. Scientists are not magicians.
Research brings new knowledge all the time and at an accelerating rate.
However, our profession is the repository of existing knowledge in the field of food science and technology, and includes the researchers expanding the boundaries of that knowledge and the experts applying it for a safe, wholesome, nutritious and attractive food supply for the public benefit. More information can be found at US FDA Center for Food Safety and Nutrition and at the International Food Information Council .