Because the new corporate masters don't want to give the working folks the impression that they are important. That notion apparently died during the 90's.
Yes, that's part of it.
IMO, companies used to do this to:
1. Make employees feel proud of their work,
2. Attract talents,
3. Attract customers,
4. Document the company's history,
5. Convey the values of the company.
But things have changed.
1: Most companies do not care anymore. Turn-over has become the norm rather than the exception.
2: These days, you'll attract "talents" with loads of cash and benefits. Offer loads of it, and the fraction of people that will reject an offer based on their perception of the company will be noise. Unless maybe you're an absolute star.
3. Sadly, most customers now will fall for the hype (brand name, alleged technology) and the price tag. Products could be designed by a troop of monkeys for all they know, it doesn't matter.
4. Probably seen as a net loss of time. That has been largely replaced with a few colorful PowerPoints, made in a couple hours by marketing people.
5. Not sure how much this matters anymore?
Sure there's also the point of documenting internal processes, pieces of schematics, etc. That's something companies had no problem doing back in the days, but these days, many "hide" as much as they can, for fear of competitors getting ideas.
But as pointed out above, I think a lot of it has to do with the balance of power between companies, their employees (in particular engineers) and their customers having changed drastically.
In the end, companies used to do that because they considered it was useful for them. Now they consider it is not anymore. We just need to understand why.