many reasons. if climate changes, wars, etc force an indigenous group to move, over time nature just covers what came before. or be built over after being destroyed by another culture. cultural lore stays with the group even as it moves. it evolves like a game of 'telephone' unless it is written. and even then, until the printing press, the hand transcribing produced changes in translations and interpretations. making stories even in the bible open to interpretation. was it the red sea, or the reed sea, that parted? for examle.. the topography has changes that make it difficult to use the bible as a rand mcnally road map.