The reason is regardless of how much 'leave' you have its easy to get fired. If you fear for your job then it's better not to take too much leave. And managers have reason to make sure people fear for their jobs.

For another thing I don't think it is ethical. Specifically it is not fair towards people who do not have families. The loneliness we have to suffer is already punishment enough. We die earlier because we do not enjoy a happy life. Yes mothers should have a leave, because of simple necessity. But I don't think we should force people like me to compensate for fathers lack of will towards using their vacation days to care for their child.

Edit: of course medical emergencies are a different matter. I don't want to give the impression I want people to continue working if a dependant requires their care.