Question

Why vedas had to be
listened to be remembered

Answer 1

Answer:

were all composed in verse form - not a single volume of prose was ever written because verse is easier to memorize than prose. Books are subject to loss and destruction so Vedas were memorized in order to ensure their continuance.

Answer 2

It was because in those times there was no writing systems . Even though it was written down after a few centuries, many of them did not survive the harsh centuries.