This is the kind of stuff Wikis (and wiki-likes) were made for. It's so frustrating when trying to find some references and stumble across the answer on random blog posts.
They self document what they did at work in their own private time. Companies engender this behaviour by calling them "heroes" or "MVP". Particular product/technology gets exposure, free (as in beer) advertising, and similarly as free digestible documentation (project walkthrough) from your staff.
If you forced employee to publish on a wiki, it would feel far too much like work. Having it on a privately owned public forum gives a sense of personal ownership and development.
It's no fun if you have to go through a multi step technical, managerial, docs team and legal review process where PM makes you include/exclude what they want to turn it into a marketing doc, your manager shuffles your priorities, the docs team makes you dumb it down, blow up the word count and remove external links and convert to .docx that will only be available to registered enterprise customers, and then finally legal tells you you can't write any of this anyway.
All of these concerns are orthogonal to whether or not these employees are expected, tacitly or otherwise, to do this necessary work on their own time. There's no reason Apple and Google, of all the companies in the world, can't set aside time for their employees to write documentation, and there's no reason they can't organize their bureaucracy in such a way as to not inhibit this creativity.
If you forced employee to publish on a wiki, it would feel far too much like work. Having it on a privately owned public forum gives a sense of personal ownership and development.