This is not simply due to their inability to digest them. Nor to training. Their body wisdom guides them. But we can trick them into eating corn and soybeans by adding the flavors they like and thereby defeat their wisdom the same way we defeat ours. The liver is deluged with the same set of pollutants time after time and never gets a rest.
Humans still obey their body wisdom about varying their meals.
If the liver is absolutely unable to handle something, you are informed quite quickly with an allergic reaction to the food. It is not surprising they are getting cancer with increasing frequency, a situation where the liver can no longer detoxify isopropyl alcohol, a common pollutant in their food. Yes, they deserve pure food, they deserve variety. Table scraps would be much less toxic for them than their commercial feed. If our food was doused with sugar for breakfast, lunch and supper, we wouldn't care much about what was under the sugar either.