Animal flesh and fats are far more difficult to digest for a sick person than vegetables and fruits. Most sick people are low on stomach acid, and animal proteins require a lot of it for proper digestion.
I agree with you, corn industry is a disaster to human health. But corn has very little to do with vegetables and fruits.
Also, fiber IS essential to human health. Our friendly flora feeds on fiber mainly, and it is well known that 80% of the immune system is located in the gut. A healthy gut means basically a healthy body.
Fiber also binds to bile acids, preventing its reabsorption.
I'm not a vegan myself, but many have lived long and healthy lives on a raw vegan diet.