Because Apple want to control the quality of the user experience.
Right now Apple Hardware and Apple Software are tightly interlinked. Apple can do things in software because they design 100% of the hardware, and vice-versa.
There's a lot of stuff Apple do in terms of quality of experience that's only possible because of that tight integration. Stuff that would be impossible to do with generic hardware and generic software.
But equally Apple know that if you buy an iPhone, you are dependent on a carrier for service. Like the rest of the world, it has probably not escaped Apple's attention that the quality of service you get from the carriers is becoming rapidly worse by the day. As a result, an Apple user's poor experience with an iPhone might have nothing to do with Apple but everything to do with the carrier.
Having carrier level integration with iPhone could also open up further user experience improvements (e.g. maybe they could do clever stuff at carrier level to help preserve battery).
Don't even get me started on the Apple TV. I've never been able to reliably use the remote (physical or app).
In my experience, mobile connectivity just keeps getting better and better. I have lived on both US coasts and traveled to multiple developing countries, and even in places like Zimbabwe I had pretty great connectivity considering where I was.
To clarify ....
When I spoke of "quality of service" above, I didn't mean technical service (although for some carriers it can be a factor), I meant customer service.
I think you'll struggle to find anyone who will say their carrier's customer service has improved in recent years. Carrier customer service, irrespective of what country you live in, is generally globally accepted to only be in the "awful to non-existent" range.
Even with several friends/family on our 10-line plan (to share costs) - it's been a breeze to maintain.
you nailed it.. they are concerned about the "quality of the user experience" and not the billions of dollars at stake?
Is this why Apple artificially limits which machines can be updated to MacOS as well, to ensure the quality of the user experience and not to drive sales?
New term: "Vertical Monopoly"