No one 'observes' thousands of years of human behavior. But if you mean simply learning about historical human behavior, then such history has in it plenty of conflicting behavior. So who gets to sort out all this conflicting behavior, categorizing and systematizing it? Does this 'observing' also apply to historical beliefs and opinions? Why would observing the general consensus of historical morality be valid to us, while the general historical consensus about the existence of God or gods not be?
Your standard is ultimately more arbitrary than a ghost.
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