Because for a long time in many societies women and children were seen as property of men and corporal punishment was seen as appropriate well after it was decided that the state could not do the same to men who were convicted of crimes. Eventually most societies have started to treat physical violence against wives as a crime, but lag far behind on children.

A huge chunk of the US still doesn’t prohibit corporal punishment for children, and everyone can probably guess most of the states that still allow it.

https://en.wikipedia.org/wiki/Corporal_punishment_of_children_in_the_United_States