The documentation they have may not be complete or in a form they are willing to release (e.g. because it mentions future products)

They also may be reluctant to release it because of fear of litigation. They may do something that’s patented, similar to a patent, or deemed patented by someone else, and releasing documentation makes it easier for others to find such cases.

Also, in the end, there’s dollars. Even the simplest process to release documentation costs money (and theirs wouldn’t be the simplest for the reasons mentioned above). They may think there’s not enough money or reputation gain in it for them to do this (and for reputation gain, they would have to keep doing it)

Trying to take a non cynical take: at a business level Apple's hardware business was growing while other makers were having a hard time, and I think with their current advancement in processor tech and sheer production quality, they have captured most of the market they were probably expecting to capture.

At this point, trying to expand the user base through sheer hardware improvement or trying to include fringe groups (windows dual boot users, linux user) probably has diminishing returns.

In contrast, service revenue, app revenue and accessories like air pods, the Apple Watch have a much better ROI.

What I'm coming at: they have little incentive to work hard to expand the user base, and a better ROI on expanding revenue from users of their software platform. So I'm not holding my breath on Apple actively helping for windows or linux support anytime soon.

So what you are saying is that engineers at Apple have nothing to say about this?

They do, demonstrably, have some sort of say. The M1 Macs have had deliberate support for 3rd party OSes since more or less day 1 (just not exposed/advertised), and Apple made some early changes to enable Asahi. Some recent comments by marcan on the topic:

https://twitter.com/marcan42/status/1554395184473190400

I don't see engineers having a say here. Instead, I see a lot of praying, begging and hoping in that thread.

What do you want, engineers explicitly coming out and saying they designed the system to make this possible? Oh wait, they did.

Apple barely releases documentation for their SDKs they expect developers to use.

They are one of the worst lock-in minded companies. Making their hardware as closed as possible fits that.