We've decided morally, that killing is wrong. So if killing is wrong, but we have to keep killers out of society, then we've got to put them in a place away from society. Somewhere along the way, we decided that killing isn't the only thing that requires you be separated from society.
You haven't committed a crime, therefore are free to succeed or fail at life all on your own. Society hasn't judged you, therefore society hasn't seen the need to take care of you either.