They have incredible amount of cash reserves. An investment in Linux drivers would go against their mission of OSX penetration. It has nothing to do with being unable to do it.
If you could 100% compartmentalize this, have people working on it independently, maybe you could minimize impact, but how can you justify that spending?
They know the open-source community is capable of working to fix problems like this.
The open-source community can do it. Look, we got Linux on the PS4. This isn't even hard by comparison.
It's about control. Apple doesn't want YOU to replace OSX, they want to do it themselves with something they control, not let you gain control.
However thanks to the razor thin margins and the desktops being replaced by laptops with docking stations, they are a dying breed.
Investing in Linux would not be a problem it just means that is becomes just a tiny bit harder to do whatever they want on the hardware side. I'm surprised they still maintain Bootcamp.
Apple prefers and has done a lot better with smaller, more focused teams.