It's done for security because the screens are primary input devices and linked to the biometric sensors. Without protections for genuine components anyone could replace your screen with one that had a hardware key logger and compromised biometric sensors. The security of the system is as weak as the weakest component.

Your phone has access to tons of personal data, for some people just about all of their personal data. It's in every user's best interest to have secure hardware since it's inherently mobile and easily lost or stolen, easier than a desktop locked in a house.

But no I'm sure it's just some conspiracy to make iPhones disposable. That makes way more sense.