Maybe you've never needed to open your iphone because it would serve no purpose. If I could upgrade my old iphone by taking it apart, I wouldn't have bought a new one in the first place...
You don’t have to buy a new iPhone to replace the battery, so presumably you’re imagining upgrading the processors, camera, sensors or some other aspect of the iPhone. Those components are so tightly integrated and custom fitted in each iteration of the device that it is technically impossible to “upgrade” them independently.