Also, it must not be forgotten that there is a lot more than writing the pages themselves. On a document this big, you need to properly organize and index it s

Also, it must not be forgotten that there is a lot more than writing the pages themselves. On a document this big, you need to properly organize and index it so that the maintenance personnel would be able to find the relevant bits easily. that's a lot of editing job required, and a lot of coordination (to properly reference and label each subsystem for example). Also, this document requires a lot of reviews and a lot of the maintenance procedures needs to actually be performed to actually check if they are clear enough to be executed reliably by operators/maintenance personnel. All that is a lot of work, requiring a significant amount of man hours by skilled people and hardware and tooling available for validation.