Maybe not enough foods that meet that requirement is available to feed all of the us?

"In the last couple of centuries humans have done a strange thing: we’ve dug the biggest pits, the deepest holes, and the longest tunnels the world has ever seen, all to find the most insidious and subtle poisons known to our mammalian bodies, remove them from deep inside rocks where they had lain sequestered for eons, and concentrate them in the places where most of us live. We’re starting to think this maybe wasn’t a good idea." https://www.resilience.org/stories/2014-08-11/using-plants-t...