you'd imagine they, upon acquiring said expertise, would write it down so that it is permanently available to others. What happens to the expertise if the person with it dies in an accident or old age?
Some institutional knowledge is just knowing where it was written down.
Some of it is trade secrets and so when the company is wound down it's sold as IP to another place.
Some expertise is contextual to the actual build of the equipment. Having a manual that said replace this O-ring every week in a blower or you'll suffer failures may not actually be expert knowledge anymore, It could be all that is stopping you from losing a million dollar piece of equipment or it could be what's causing you to lose productivity for an hour every week in a pagan ritual to the sacred maintenance document.
Being an expert and being able to chronicle that expertise are not just separate skills they're separate tasks. They might not know what is worth highlighting or they might prefer to read a book than to take on the extra task of writing it down.
You do write it down for exactly that reason, but it doesn't make it permanently available in the way that you describe - much of generational knowledge transfer happens by living and working side by side, not just reading books that the elders wrote.
you'd imagine they, upon acquiring said expertise, would write it down so that it is permanently available to others. What happens to the expertise if the person with it dies in an accident or old age?