I probably wouldn't use it without that. It's one thing sending my shitty code to be trained on, and another thing entirely to give these companies access to my personal life and information.
I bet it's Apple and not the user.
I suspect if you paid apple enough money, and were willing to prove that your personal Private Cloud Compute did meet their requirements, it wouldn’t be impossible.
The hardware isn't a real justification, just a convenient fig leaf.
When you have to image a highly irrational reason to explain why groups of people do the things they do, there's a decent chance you just don't understand their perspective. They may be acting reasonably rationally from their own perspective. (As you said yourself: "I honestly don't understand...")