And reasonably so. Partly, this is because the "best diet" at any given time depends on practical factors like availability and affordability. Both of those have been rapidly improving, I think, for much of the world. I can buy food today that, to my grandparents or even parents, was unheard-of or only-eaten-by-kings.

e.g. the value of quinoa or blue-green algae in a diet probably makes sense to study now, but it probably didn't 50 years ago