Because children have been treated like disposable pieces of shit for centuries. Listen to Dan Carlin's Hardcore History on the treatment of children throughout modern civilization. It wasn't until relatively recently that parents were barred from slaving their children out working in coal mines and hard, extremely dangerous jobs to support their family.
Children were forced to be the chimney sweep. Some man would stuff naked children up the chimney and drag their naked bodies against the hard walls of the chimney cleaning out the soot and debris that stuck to the walls of the chimney.
People still hit children, because children are seen as sub-human. You will see the same logic from battered wives that you see from adults trying to justify the abuse they received. "I deserved to be spanked, because I was a real shit head." Both relationships often are born out of dependency. Both often exist in a relationship where a stronger person beats up on a weaker individual.
You will not often see public, majority of people speak out for the spanking/beating of small animals like dogs in the name of discipline. That practice has for the most part been set aside as an example of humanities barbarism. Yet, you will still see people support hitting children for reasons like "that's what my parents or grandparents did and I deserved it." There are entire generations of people suffering from the same feelings that battered spouses suffer through, yet we normalize this as part of our culture.
It's not about being politically correct. You wouldn't hit a dog, you wouldn't hit a co-worker, you wouldn't hit your mother, you wouldn't hit your wife, yet hitting children is fine despite research saying that it doesn't work. This is without considering whether it would be humane if it worked at all. I think children deserve to be treated with the same respect that all humans deserve no matter what their age, gender, race, or health situation.
