This is what I have come to understand (I could be wrong< I have been wrong before!)
I think it is that grains have developed defenses to avoid being digested. The husk and phytic acid (phytate) seem to be the issue.
"Phytate is not digestible to humans or nonruminant animals, however, so it is not a source of either inositol or phosphate if eaten directly. Moreover, it chelates and thus makes unabsorbable certain important minor minerals such as zinc and iron, and to a lesser extent, also macro minerals such as calcium and magnesium."
There has been discussion of the phytates even removing nutrients from other foods and passing them through the intestines without absorbtion (possible cause for anemia?).
Basically seeds have developed so they can be eaten and pass through the digestive track (without losing nutrients) and then excreted with the ability to germinate.
Please correct me if I am wrong.