That is a very good question, Brother. I have actually been pondering on that for almost three decades already.
This is going to sound very dark, but if we look at the "official" explanation of what sets humans apart from all other animals on this planet, then the claim is that humans have sapience while other animals do not. But then why are those other animals, who lack this sapience, not as depraved and evil as humans? Could it then really be true that evil is indigenous to the human psyche, and that it then takes a considerable amount of good — which has now begun waning again as of the 20th century — just to keep this evil subdued and balance things out?
It's a painful idea to contemplate.