Because we have more kinds of organisms growing in our mouths than any other animal. Because we eat more kinds of things than any other animal on earth. Man has mastered all kinds of situations, and has grown to a very high degree of dietary versatility. They must have very precise types of food, at precise times. Some more than others, of course.
Chickens are one of man's closest domesticated friends.
They, like dogs, have grown to eat many of the foods of their keepers. If you keep them penned up most or all of the time, you will also need to make sure they get . . . Chickens won't always search far for food or water. Sometimes they won't even go around a see-through fence to get it. And they need lots of water, especially when laying.