Because why? What would the motivation be? They are a company that goal is to make money for their shareholders. They purchased NeXT which was built on MACH+FreeBSD which was released in Sept. 1989 (work started in 1985), 3 years before Linux 1.0 was released. MACH+FreeBSD was a stable working platform for Apple when they acquired NeXT in 1997. There was zero reason for them to move working code to Linux 2.x (2.0 shipped in 1996). Heck I had a NeXt setup and a Slackware setup at that time and NeXT was better hands down. Today the kernel they used is tightly coupled to their HW and they have control over it for HW features like the T2. Why would a $2T company put themself at the mercy of a kernel maintainer to take their patches? The vast Linux ecosystem is of no use to Apple in most cases.