Why is it widely encouraged to parent our kids in ways that would be abusive if done to an adult?

The sole purpose for punishing our children is to teach and reinforce that improper behavior has adverse consequences. By adulthood that lesson should have been well-learned, but we do have things like on-the-job disciplinary actions (including being fired) and a court system to deal with those adults who need to be reminded. Children are not miniature adults, and it is the responsibility of all parents to teach their children how to behave properly in society.