Sure, but Apple has been researching and applying privacy-preserving techniques like differential privacy. And they do a lot of their machine learning on-device

Sure, but Apple has been researching and applying privacy-preserving techniques like differential privacy. And they do a lot of their machine learning on-device (e.g. face unlock is entirely on-device). Really the big outlier here is Siri, I'm sure they could make Siri better if they were willing to hoard data, but Apple has demonstrated that they value user privacy much higher than the incremental improvement they'd make with Siri, and violating user privacy would likely hurt their business a lot more than making Siri a bit better would help.