Why do some restaurants (regardless of what type) not allow outside food, drinks, etc...?

Restaurants are in the business of selling food. The definition of a business is to make a profit. I know that beer, pop, etc are cheaper at the store, but thats where a LOT of the profit in a restaurant is. Also, if you bring Popeyes Chicken into a KFC, you are boldly advertising their competition in there own restaurant. It's rude & wrong.