Probably because they thought physical discipline was an effective method of training at an impressionable age but was obviously only a method of punishment once you reached adulthood. Or maybe because enough of the people who made the laws could imagine themselves on the side of the prisoner, but none of them would ever be in the position of being a child again so there wasn’t enough self interest for that law to be different.

Basically probably no good reason, but just a misguided vague ‘nip it in the bud’ attitude carried over the generations.