True, but we also are designed to eat one thing at a time whenever we could find it. There were no plates, dinner prep, or freezers. We found, we ate. Complex carbs are ok to eat too, but sparingly. When we found them, they were small bits of grain that we would eat slowly because they were so hard to gather. Grains would usually be a last resort when there was no game, fish or fruits.
Today's grilled cheese sandwich or cereal with milk cannot be processed by our digestive systems.
Can you imagine what people would think of someone who walked into a field of barley, ripped off a few stalks and just started gnawing.
People would think they were insane.