Because some of the restaurants are just too expensive and the food they serve are unpredictable. If you eat in the chains, at least you can have some control of the prices or some idea as how the food should taste like.
Because people outside of NY are smart enough not to pay triple the price because a restaurant has a fancy Italian name when the food isn't any better. If you want authentic fare you certainly do not go to New York unless you want to get mugged before you reach the entrance. The best authentic food is actually found in California.