Why can't I just continue feeding my toddler formula milk?

My friends tell me that he needs to switch to regular whole milk instead of formula milk and to feed him solid foods. However, he is a picky eater and whole mlik doesn't have a complete nutrition that formula has, in addition to formula's lipids, proteins, and vitamins and minerals. So why can't I just keep him on his formula diet until he gets old enough to want to eat a balanced meal? He is 20 months old right now.