During those many long nights wrestling angrily against arcane code, I made a promise to myself that anyone working with my code would never have to guess what was going on. I'd leave a trail of breadcrumbs allowing them to follow my logic. I merely considered it polite behavior and a common courtesy to an unknown fellow human being. This 'lack of documentation' is not a problem unique to the IT industry. We all know documentation is a good thing.
Documentation is to a corporation, what long term memory is to an individual.
Without documentation an organization's technical accomplishments and information resources are continually eroded by the passage of time and employee turnover. Consider it this way. Turnover, combined with poor documentation standards combine to create a nasty form of organizational Alzheimer's. There is nothing difficult about documentation. It's merely a record of what you've done and why, how it works and how often, who uses it and when, etc.