Cats are domesticated animals, heavily influenced by human society. Nonetheless, the cat is a cat. In a very distant past we may indeed share a common ancestor as mammals, but otherwise the cat's physiology is as much an extreme when compared to ours, as day is to night. If we want to understand the dietary needs of cats, we must look at cats as they live in the wild, and not at how they live as part of human civilization, and thusly the foods they adapted to eat.
Grain is man made - grass seeds, enhanced in size through selective breeding.
Before humans discontinued their nomadic lifestyle and settled as farmers, there was no grain. There where only seeds - available to wildlife for the take merely seasonal. Advocates of the use of grain in cat foods argue, that these foods would naturally be part of the cat's diet as the stomach content of their prey - rodents and birds. Yet, these species too, where first introduced to grain merely ten thousand years ago, and that only in the presence of far apart human settlements. A truly natural diet for rodents and birds is not based on grains, but on insects and fruits, as well as greens for rodents.