Uh, because it is.
12,000 years ago, Rig Veda; 8,000 years go by, nothing (including btw no trace of the Rig Veda or any other texts in all of India, other than the Harappan script, oldest known about 3500 BC, but which remains undeciphered, but is nearly uniformly recognized as holding an unknown agglutinative language, which the Indic languages obviously aren't), then all of a sudden, written versions first appear when, surprise, writing makes its debut in the subcontinent.
Michael Wood, in his "The Story of India", does some of his best presentation in the first chapter of that series, but he goes off on his usual "the west is awful and doomed" path throughout the series. Still enjoyable. He makes the obviously untestable and quite ridiculous claim that the oral transmission of epics has been perfectly accurate all the way back to the era when humans were mimicking bird sounds and other animal noises, before humans started speaking on their own.
While oral traditions are clearly what we still operate under, for everyday life (don't eat that, it's poisonous, etc) it falls flat on its ass in the consistent and accurate transmission of prehistory. Too bad, because I've always liked it. Nice and malleable.
‘agglutinative’
Damn you, you’re always pushing on the edges of my knowledge.
oral transmission of epics has been perfectly accurate
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I believe that worked quite well for a very long time with the Iliad before Homer wrote it down. There are long traditions of stories that were transmitted verbatim back when people used to have aural memories which we have since lost.
“writing makes its debut in the subcontinent.”
That have been found - there are many underwater cities off the Indian coastline that are only partially explored, if at all, likely dating back before sea level rise during the end of the Ice Age or end or the Younger Dryas.