Why are words which have been spoken 1500, 2000 or 2500 years ago still repeated today and still helpful?

Why are words of Shantideva, (685 p. Chr n.) Christian and Buddhist teachings repeated even today 1500 years, 2000 years, 2500 years after they have been spoken for the first time by their authors still repeated today and helpful? Is this because these where authors which where alive, and said thing nobody ever has spoken …