For the simple reason that there is a huge software library targeting OpenGL that is being dumped. Much of it my industry still uses.
As far as me, I've stated what I would like to be able to buy. Since it isn't something they intend to sell, I've bought my last Apple system, and will make sure they are never purchased for work purposes. So, easy fix. I'm in a market they care little about these days and has more or less dropped them in response.
Again, Linux on their hardware would be great. MacOS on their hardware I won't buy. Not sure why this required so many pages to state. When I bought into it (MacOS) the focus was a bit less walled garden. Simple. A vendor changes something I rely on? I change vendors. I don't work for Apple, do you?