Probably because of the way it is served giant quantities, overcooked and drowning in acidic slop like marinara sauce. Most humans can probably process grain when eaten naturally: raw, chewed thoroughly since it is raw, and eaten alone as an alternative to starvation since nothing else is available. Like most other diet maladies, the problem turns out to be culture not ingredients.
/johnny
Please, ever heard of stomach acid? While it best to send your food down in smaller parts, you could eat a big enough "raw" piece of food and your stomach acid would still tear it apart. Although, admit it may give you a stomach ache or acid reflux. Still, the acid will desolve it enough to flow through the intestines.
BTW, although there are probably too many extra ingredients in our foods (most for preservation), "raw" food is a fallacy. People have been living longer and longer if they just eat a normal/balanced amount of even processed foods. That's not counting medical care.