Because they didn't have common written histories?
there were Indian civilizations that had written histories, including bark leaf books - unfortunately, few survived the Conquistadors and time.
And it has been only recently, time wise, that scholars cracked the code and are able to translate...
Other clans/tribes across the lands had their own methods of keeping their histories, including oral history. A history is a history = and therefore, not prehistoric....even if someone would like to paint it so.