there are a few very important reasons why the druids taught their students in spoken word only. this wasnt because they were too dumb to write them down, it was for a few reasons I will discuss.
One reason is for their teachings to only be taught to students. laypeople and others can not easily take or copy their papers if they are never written down. how many times have you seen people mistranslate what was said and used certain wisdom for evil? Ive seen it many times
Another reason is to make sure that the teachings are taught heart to heart from teacher to student. This has the effect of keeping the community/relationships and body of teachings together rather than splintering off. this keeps the bond between student and teacher close.
third reason is take the Bible for example. How many times has it been edited and changed for personal reasons by one monk? many times. when you learn from face to face, many flaws reveal themselves in the open where it does not on paper. the teachers life speaks for itself
how many people take the word of the bible and use it to insult or mock christianity? it gets ruined in the process. Jesus himself taught not to throw your pearls to swine, and writing down his teachings shows why you shouldnt do that. the swine of the world has tarnished it
How often do words change their meaning over the course of 20 years? what a sentence meant in 2000 might be said completely differently in 2020, but yet the spirit of the teaching is the same. by the time we read them, the original meaning is lost.
collected texts end up centralized back then in libraries and now on google, where the one collecting gets too powerful. it is impossible to centralize spoken words(without recording them). decentralizing anything is good because it makes it uncorruptable.