antediluvian age and common knowledge
A fraction of the top 1 percent of the "successful" Warlords could read. Even the successful could read still had to depended on word of mouth and hero worship for knowledge.
Before censorship could manipulate the narrative, teaching old world peasants to read was called Treason and subversion.
Educated salves become self aware so we can't really blame the control freaks who cull uppity slaves.