>Doing the same thing to an adult can get you an assault charge, or a punch in the face.
Kids aren't adults and they can't reason with logic or words. Therefore, if a child does something like bite, hit etc. (which they do often) an equal response teaches them immediately that committing those acts leads to a negative outcome. Of course, modern mentality is that this is an awful abusive way to treat your child. I disagree, I don't think a spank is akin to full on abuse and there are times when it's appropriate.
Additionally, adults that do act the way children do and start violently attacking people often get a quick response either from the individual they were attacking or the cops. Maybe they never learned consequences for their actions when they were young?
Have you ever been around a group of young men or boys when someone started something? Typically the quickest way to end it is a swift physical reaction that immediately shows you won't tolerate stuff.