Well, I'd say it's because the aliens posing as the 'elites' at the top level (there are plenty of human flunkies though) don't want us to know the truth about any of our history and consequently give that 'science' carte blanche, to make up versions that fit their agenda.
And I'd say, it took those 'gods' probably around 2,000 or so years to adapt us, since they manipulate energy at the molecular level and so can adapt DNA stuff and yes, they considered their need for us (just the body part) critical.