For example, from "my house is on fire" (what is) alone does logically not follow "I should try to extinguish the fire" (what should be), without a quiet assumption something like "my home and belongings should not go up in flames" (what should be).
Ethics are obviously within the category of how things should to be, and thus it cannot be derived in its entirety from the state of the world as it is.
Thus ethics must be derived from some other notion about how the world should be. If you take account of the sources of our fundamental notions about how the world should be, you'll find they're all some variant of emotions, or possibly something like aesthetics, which unfortunately again cooks down a desire for that which is pleasant to the mind and senses.
All other notions about how the world should be are so-that something else, which if you keep following the reasons so-that, all land in emotions. For example, the reason why we seek things like truth and justice is because we like how they feel, we feel content, as though things are the way we think they should be. We dislike lies and injustice because they make us angry when it happens to others, sad or indignant when they happen to us.
If we did not feel this way, surely we would have no reason to prefer justice to injustice, or truth to lies. It would by definition be all the same to us.
Alternatively, one may argue that God has decided how things should be. But that is just replacing human emotion with the moods of God. God may get angry if you marry your daughter and ransack the temple and will be happy if you offer libations. An ethics based on piety is a system of ethics based on whatever makes God happy.