Or because, you know, they make money selling that OS. And they understandably have no desire to make the entire thing GPLv3. And they also have countless contractual obligations to other vendors whose closed-source code is also bundled in OSX.
I find this attitude kind of troubling around here when every other day there's some "open source" project that has had to change licensing because insert cloud provider is stealing their lunch making it impossible for them to make a living. It turns out that someone has to pay the bills, and giving all of your software away isn't a great way to do that. Outside of Redhat, barely anyone has figured out how to make a business out of giving away software, and I'd argue the sale to IBM shows that even Redhat saw the gorilla that was AWS standing in the corner and decided it was time to throw in the towel.