The problem with rewards and punishment in the afterlife is that they happen after death, and there are no witnesses who report back to the living. No-one has returned from heaven or hell to tell us if these places exist, or if the punishments and rewards there fit the deeds. There have been no studies of people claiming to be reincarnated, to check whether each new incarnation corresponds to the merits of the previous life. Indeed to substantiate fully the karma theory, interviews with animals would be required. Ethical systems whose sanctions are invisible and have never been witnessed by any living person, stand on shaky ground.
Human sanctions are quite adequate.
Order is not kept in society by fear of God. These sanctions work for the vast majority of people in almost all countries with functioning governments. Traditional religious ethics are inflexible and outdated. Revealed religions like Zoroastrianism, Judaism, Christianity and Islam do prescribe some clear and unambiguous rules. If, as they usually claim, their scriptures were authored or dictated by God, then the commands in them are God's own commands.