This is never taught properly in textbooks for a variety of reasons. For starters people don't want to give up their personal bias, their cultural customs, and they definitely don't want full responsibility for their good and bad decisions etc...
Furthermore, schools don't want to take a hard line on subjects like this because there will always be people by the hundreds of millions incapable of, and unwilling to be objective, unemotional and accept the facts in this department. Not only is the subject of diet highly emotional (a very odd phenomenon where people's identity becomes what they eat or don't eat) alienating that many people is flat out bad for business.
Emotions aside, there is an unseen web in the big publishing world controlling the information gatekeepers of the system are exposed to. They don't want the truth getting out, they don't want objective, unemotional, definitive facts on nutrition based on Anatomical design becoming mainstream and uncontested because then people would be impossible to control. If the objective truth about diet was out and followed, both supplement and drug companies would take a huge hit to their profits and therefore have a vested interest in confusion remaining the status quo.
The reason it works and nobody seems to care or notice, including amongst grads of these programs? There are 3 primary reasons:
1) They don't want responsibility all that much more then the general public, as nobody wants to live perfectly or be held to that standard.
2) Subjects are taught in Silo's in school so the dots never get connected
3) The education system rewards, regurgitation without question, not independent thought and it tends to attract people accordingly.
Don't get me wrong there are many nice, intelligent people with great intentions that graduate from all the above, many of whom I call friends. This is not personal and I suspect there are many humble enough to tell you about the shortcomings of formal education and credentials.
If you doubt the education system is flawed across the board allow me to teach the remainder of this article in pictures using common sense.
Some Dr's say this is healthier than.... This?
Some "Experts" believe he should be doing this instead...
Some "Experts" believe you can skip diet and just eat pills, potions, tinctures and lotions. They only debate whether or not it should have come from a lab or Nature.
If textbooks were teaching things properly could the diversity of opinions so wildly across the spectrum even be possible? Of course not! Take the pills out of the equation and justify while remaining objective, and logically consistent how this mess (which came from the pot of bones humans are both unwilling and incapable of eating above) be superior to fruit which is constantly vilified. Gag!