Parents seem to be on this power trip with their kids. It's the only explanation for the ferocity with which these parents cling to the concept of spanking.
Children are young, less experienced, less knowledgeable and can be defiant, impulsive and downright annoying and often even infuriating.
When you have someone that you rule over, who knows less than you and who defies and annoys you, you feel completely justified in hitting them.
You say it's to teach them a lesson. You say it's because they can't learn any other way.
You say it's your right as their "ruler."
Let's grow beyond this and act benevolently to our subjects, eh?