Yes, you cannot do that and I would prefer that they enable these features too. But really, for most lay people it is better, safer and convenient to just use

Yes, you cannot do that and I would prefer that they enable these features too. But really, for most lay people it is better, safer and convenient to just use Apple selected method - to connect everything with one account/Apple ID. And I think that *primary reason* is better product for users. Overall it is easier to control user experience if you tightly control whole platform. I am not sure if it could be even possible to create broad and well integrated platform similar to what Apple has now, with general opennes. Is there an example of this in any other tech domain? Counter-example: Linux desktop is nice, functional, very open but so complicated and fragmented ... and it never took off and it is staying as a niche among desktops so far.