I bet that this is more a business/philosophical problem between NVIDIA and Apple than a technical one and NVIDIA GPUs would work with drivers.
The GPU issues with earlier MacBook Pros (8600M GT, IIRC) aside, I believe it comes down to business and ego.
NVIDIA is a company with a dynamic leader who built the company from the ground up and is a leader in its field.
APPLE is a company whose dynamic leader built the company from the ground up and is a leader in its field.
Both companies are used to being the dominant voice in any room or at any negotiating table. This can lead to a clash of personalities and egos, even outside of the CEOs and Executive Suite.
NVIDIA is very protective of its technology, IP, secrets, et al.
Apple is very protective of its technology, IP, secrets, et al.
Both companies believe the way their way is the better way forward.
NVIDIA wants its proprietary CUDA PCP platform and API product to dominate the market, especially AI and machine learning, which in turns sells more NVIDIA GPUs (hardware).
Apple wants its proprietary METAL 3D GPU and API and Core ML product to dominate the market, especially AI and machine learning, which in turn sells more Apple hardware devices. Apple does not want CUDA to dominate this market.
NVIDIA wants to maintain its leadership and its secrets.
Apple wants to maintain its leadership and its secrets.
NVIDIA believe the GPU is central and in some cases, more important, than any other component of the typical computer, which is in entitled to believe, even if it is a bit off the mark.
Apple believes the GPU is simply one puzzle piece in a larger strategy and therefore not especially special or noteworthy to the other pieces, especially if they are buying it from another vendor and not engineering it themselves. Therefore, Apple believe NVIDIA is like any other LARGE component vendor they partner with (AMD, Intel, Qualcomm); important, strategic...but not to be trusted anymore than is absolutely necessary for them to know only enough to get the job done and the hardware built for Apple to sell and reap the rewards.
Apple views NVIDIA as a competitor, which means they do not consider it a trustworthy strategic partner and, in the end, it does not serve Apple's needs in the way Apple wants. Therefore, NVIDIA is kept out and barely even acknowledged as existing.
Just my 2¢.