It's because they do not integrate evolutionary thinking. This leads to the eating of highly processed and packaged foods, the eating of non-foods, the eating of anti-nutrients and toxins we didn't evolve to eat (like grains — chiefly wheat and corn), puts us in chronic caloric depravation, ignores gene expression pathways critical to optimal health, overtrains us, focusses on weight loss instead of fat loss and lean gain, and on and on.
So, why is an evolutionary approach so essential?
From Loren Cordain's free Paleo Diet Update.
The graph below illustrates the magnitudes of the time our ancestors ate a Hunters and Gatherers' (H-G) Diet versus when our ancestors consumed a Mass-Agriculture Diet. The specific times used in this graph are 2,000,000 for the H-G Diet and 10,000 years for the geologically recent Mass-Agriculture Diet. Although exact dates and amounts can be argued, and would change some among different ethnic groups and regional histories, the graph would always look very much the same – because regardless of the specific dates you utilize, it always would very definitively involve magnitudes of change difference.
Bar graph illustrating a ratio of geologic time: 2,000,000 years vs. 10,000 years. These times are good representations of the magnitude of time of the Paleolithic Era foodstuffs of our ancestors as compared to the time our ancestral lineages have been on a Mass-Agriculture Diet.
It is startling to see the Mass-Agriculture Diet as a nearly flat, non-existent bar. In a mathematical sense one could almost say it is approaching the inverse of infinity … or that it is "infinitesimally small" in comparison to our earlier foodstuffs. It is more than a full 2 magnitudes smaller. As a decimal ratio of 2,000,000: it is .005.
While we can continue to debate (and we should) the exact amounts and rates of change in human physiology and the dietary amount of animal products vs. fruits/vegetables, etc. – an obvious fact is that the amount of time we and our ancestors have had mass agriculture and industrial era food is incredibly small indeed … and not debatable.
When we talk about "evolutionary discordance" in regard to our modern diet vs. the Paleo Diet, this is what it means in one very real sense. A diet based on the way humans ate for a couple million years will lead to optimimum health and reduce the risk of degenerative disease.