This is one of those things that shouldn't need to be explained, even to 20-something tech bros who've never had a child themselves.
And they can’t understand why they keep on failing to achieve their policy goals.
PS: I don’t think there were too many <= 19 year olds signing up for hacker news in 2011.
Just to be clear hear, we're talking about parents being able to take care of their newborns and not being forced back to work after "2-4 weeks" as the OP stated.