Why do people without children have the nerve to lecture parents about how to raise their own children?

i was at a restaurant yesterday and some young punk came up and decided to lecture me on why it's bad for my child to play around the tables and say hi to people, who do these people think they are, i'm a mom i'll raise MY child how ever i want to so shut the heck up, you wouldn't know anything about children, you don't have any, if you ever decide to grow up and act like an ADULT, you would have children.