Why might Aztec contributions to the codices have included accounts of such supernatural events preceding the arrival of the Spanish

All of the Aztec codices and stele depicting Human sacrifice that we see today - were made after the Aztec had been conquered! Even at the Cenotes of sacrifice at Chichen itza, where there are so many stories of people being thrown in as sacrifices to the gods. Modern day dredging has recovered many items of gold and jade as well as pottery - "But" no Human remains. The explanation for this is that the Spanish destroyed all of the original Aztec Books and stele. Which seems pretty strange, when you consider that the Spanish were very keen to study the Aztec. Why then too, allow such a great abundance of it to be re-created after the fact. If it wasn’t right for them to exist before, why was it right afterward?