The fields of nutrition and economics do have the basics worked out. But the basics aren't things like "is eating too much salt bad for you?" The basics are a

The fields of nutrition and economics do have the basics worked out. But the basics aren't things like "is eating too much salt bad for you?" The basics are about nutrient metabolism and the human body is a very complicated (/complex) system. The basics don't necessarily have to be "actionable", and they don't necessarily need to say how the system comes together as a whole. For instance, in your example of evolutionary biology, there is a lot of debate about whether natural selection or neutral evolution is the primary driver behind species diversity. We know that both exist and both happen, but we don't know how they came together to create the species diversity we observe. Another aspect of this is that there are certain fields (namely economics and nutrition) where the popular press over-exaggerates every single paper as a groundbreaking discovery because it gets clicks. The media tries to present the idea that scientists as a whole have come to a brand new conclusion which should change the way everyone behaves, when in reality it's just a few people in a windowless room reporting on some observations they found and speculating on what those observations might mean.