If monkeys can establish what is morally right without science, why do humans need science to establish same?
Because monkeys can fail in blatantly obvious ways and they have no way to discern one from the other.
Science is a set of methodologies we have developed over the ages for removing sources of factual error, illogical reasoning, and cognitive biases from our conclusions. Do you think you are better off leaving in known sources of errors, illogic, and bias?
We used to think it immoral NOT to beat children to teach them to behave—now we know it actually does long-lasting harm. We know this through careful observation and study.
That doesn’t mean that simple empathy-driven behaviors cannot be positive, it just means the less obvious questions require more careful thought sometimes.