Do you honestly think Apple does not want you to repair your own device? They would absolutely sh*t kittens to have you buy replacement parts from them!
The problem is that smartphones are super complex devices, able to withstand dust, rain, drops, and more. In order to do that, they have complex internals, which AMONG OTHER THINGS (like heat dissipation and RF I/O) are designed to support mentioned goals.
Current gen iPhones can be dropped onto concrete at arms-length and not suffer from a broken screen or back. Think about that for a moment. Could you build a smartphone that could do that, and have amazing battery life along with desktop performance?
Note that I'm absolutely not affiliated with Apple, or a shareholder.
I feel like some folks downplay the amount of engineering that goes into the iPhone, and to a somewhat lesser extent, ANY mobile device. For many of us, these things live in our pockets and our hands. They are expected to have many hours of screen time, play games, let us read books, etc. They aren't simple devices, especially the current gen iPhones Apple has been pushing out.