knowledge is power and people don't always want to share. Maybe it's more reflective of my company culture but I've seen knowledge effectively hoarded and used strategically as a weapon at times.
It is visible everywhere. Some people hoard knowledge so that they stay important in the company. Some people hoard knowledge so that they can get more money from bug bounties. It is almost always about personal gain.
Of course there is no upside for spending time updating documentation unless it actually is part of your job description or there is legal requirement for company.
If you put knowledge in wiki, no one will read it and they will keep asking about stuff anyway.
Then if you put it there and keep it up to date you open yourself to a bunch of attacks from unhappy coworkers who might use it as a weapon nagging that you did not do good job or find some gaps they can nag about.