If Theism is True, Why Is There Moral Progress and Not “Moral Prophecies”?

If theism is true, why aren’t there “moral prophets” in the sense that they clearly perceive objective moral truths which are ahead of their time, such as someone 2,000 years ago declaring slavery is wrong, misogyny is wrong, etc.? Why do we instead observe moral progress? For example, why did much of humanity, for most of human history, believe that slavery was morally acceptable? What possible moral justification could God have for allowing people, on such a massive scale, to have mistaken moral beliefs about so many things?

If we make an analogy between God and human parents, believing in theism and moral progress is analogous to a human parent letting children believe that it’s okay to, say, hit other people, until the children grow up to become teenagers, at which point the children “discover” that assault is so morally acceptable after all. Since a good human parent would never do this, why would a good God do this?