IMHO this fail is cultural, but fixable, and is a similar symptom to feature creep or platform sprawl - people gravitate more towards creation than curation, and so you wind up with celebration of "adding more documentation", ignoring whether or not the doxs are relevant or valuable.
It's similar to some of the traps I see burgeoning PM's fall into, where they're moving things around, following or building a process, and the feeling of work makes them feel like they're getting things done, but they're really just doing something with low value for time like printing out JIRA tickets to put on a whiteboard or copying the `README.md` over to Confluence.
A few culture hacks I've seen success with over the years;
- Move private questions into a public space, and have someone pay attention to them. When the same thing gets asked a few times, don't just add the answer to the docs, but look at why the context isn't apparent and the question keeps getting asked. Fix that instead of answering the question.
- Aggressively mark deprecated docs. Make this easy process-wise. In one case we just tagged a wiki page with a tag that changed the whole styling to make it really visually apparent that it might be wrong.
- Foster a culture of cutting - this usually has to happen top-down, but once you get this baked in, it has compounding benefit across the place. Cut features, cut code, cut documentation.