They already do this. It's called federated learning and its a way for them to use your data to help personalize the model for you and also (to a much lesser extent) the global model for everyone whilst still respecting your data privacy. It's not to save money, it's so they can keep your data private on device and still use ML.

https://www.technologyreview.com/2019/12/11/131629/apple-ai-...