Prior to that, modern Macs were just PCs. Then Apple started to introduce, and then depend upon, their unique hardware.
Then the ARM migration using proprietary CPUs and GPUs really kicked it up a level. The OS relies on the hardware, which is unique to Apple now.
At least they always allow installing your own arbitrary OS.
Their goal was never to build a community around Darwin or get a bunch of people outside the company using Darwin sans Mac OS X's proprietary bits, or to get a bunch of people outside the company contributing to Darwin
> The problem was Darwin was a marketing ploy, and nothing more. Marketing thought that by throwing some source files over the wall, they could increase their market share and revenue. And they were right. You go to almost any technical conference these days, and there is an astonishing number of Mac laptops in the crowd. Prior to Mac OS X, people would have been laughed out of the building for having a Mac, but now, with Mac OS X and the magic of open source, it was suddenly OK or even cool. Managerially and culturally, Apple had no intention of becoming involved with any community outside its own walls. There were some well intentioned engineers and even a few low level managers that were duped by their own marketing department, and that was all. These people had given substantial amounts of their free time under the illusion they were doing something Right, something Good, but in the end were simply used by their own company's marketing department.
Open source software was used for the basis of macOS for Apple’s convenience, not because Apple intended to publish very much open source software.
BSD has a convenient license for these purposes and it’s why proprietary commercial products like the PlayStation 5 choose to use it.