Have you considered the reason for this is that optimal diet is highly variable, both genetic and environmental.
For example whatever a 16yo school going boy from Jordan and a 56yo recently retired woman from Nebraska ought to eat to maximize health/​performance/​happiness might be entirely different and, if switched, have disastrous effects.
But these effects could also be present in much more similar individuals and even in the same individual over time. Most humans used to eat entirely different diets depending on the weather.
Have you considered the reason for this is that optimal diet is highly variable, both genetic and environmental.
For example whatever a 16yo school going boy from Jordan and a 56yo recently retired woman from Nebraska ought to eat to maximize health/​performance/​happiness might be entirely different and, if switched, have disastrous effects.
But these effects could also be present in much more similar individuals and even in the same individual over time. Most humans used to eat entirely different diets depending on the weather.