Just look at Apple's various SDKs (WebKit, CoreMotion, ARKit, HealthKit, CoreML, and etc) and software stacks: it wants as much control as possible. Most of the time, primarily on iOS, Apple has complete and unchallenged control. In addition, particularly on the Mac platform, Apple takes existing open software standards to create its own libraries.
Going forward, especially as it wants as much ubiquity as possible between iOS and macOS, this unchallenged control and the importance of open standards will become even more important. If we just look at gaming -- never mind the other aspects of AR or GPU-assisted compute tasks such as ML -- Apple, I'm sure, wants to make iOS' vast gaming library available on macOS with as little hindrance as possible.
Nvidia have shown time and time again that they're not interested in open standards. They've been caught engaging in uncompetitive business practices in the PC and workstation market on numerous occasions. Apple wants no part of their drama.
That's not even talking about hardware features. If Apple wants to enable high refresh rate monitors on their Macs, do you think they want to waste their time and engineering resources paying for Nvidia's G-sync modules? No chance. If anything they'll take Freesync (or whatever VESA calls its official standard), absorb its features, add some additional modifications, and then integrate that into like a future T3 chip or something.
(actually, maybe an Apple T3 chip could provide a solution to integrate Thunderbolt with user-swappable GPUs...)