Documentation counts. The problem is a lot of insider and seemingly-common knowledge is never written down and is usually spread through word of mouth. Any sufficiently lengthy disruption means that knowledge is lost.
Some knowledge is hard to write down. Imagine trying to teach yourself karate from books only. It might be similar with some mechanical or artistic skills that are best taught in person.
Arguably, nowadays, with cheap recording devices everywhere, it is easier than before to produce comprehensive documentation even for such activities that are hard to write down.
