But if you're bad at tennis, all that happens is that you suck at a game and waste your time. Bad parents abuse their children and kill them. Poor tennis players can't afford rackets. Poor parents can't feed their children.
The difference here is that a tennis player is the only one feeling the impact of his decision to play. Parents are grownups and can make whatever decisions about their own lives they want, but why should society let children suffer because of the decisions of their parents?