If we really can’t know the moral value of an action, then why settle for saying its based on what humans think

I never said that we can’t know the moral value of an action. All that I’m saying is that the moral value of an action is dependent on the entity giving the value. Morals cannot exist without beings capable of having morals.

Why not just go all out and say either 1) there is no such thing as moral value or 2) there is moral value but we have no way of knowing what it is.

Because saying either of these two statements would not reflect reality. There IS a thing such as moral value. It’s just not constant for all beings capable of having morals. For the second option, there is no scientific evidence to suggest that there is indeed a universal moral constant. Hence, “knowing” that value goes out of the window.