Where's your evidence of this claim. Do you know the statistics on how often mothers hit their kids out of anger and how often they regret it immediately afterwords?
For example, 80%+ of British mothers spank their infant children before the age of 1 year old.
The average toddler gets hit 900+ times a year by their parents.
The difference is that unlike in Sweden(just an example) where all physical hitting by parents is considered the same as any other assault, in most of the world it's brushed off as punishment for acting "bad".
The reality is that most spanking isn't done as punishment, but instead re-actively out of anger when parents feel stressed, tired, etc...
You wouldn't apologize for that kind of bullshit excuse for a man hitting his wife and you wouldn't question for a minute why she left him. It's no different morally for parents.