Because the nephilim were real. They were for intents and purposes "descedents of the gods" The First pharoahs were likely descendents of nephilim. They literally believed they were descendents from the gods.
The ruling elite still believe this. Whether it is true or just a belief is really irrelevant. Its what they believe and what guides their morality and behavior. Its also why they are so keen on inter-marriage.
Ruling elites are not atheists or evolutionists. Thats the stuff for the masses. they dont actually believe it.