Those OEM parts can either come from broken devices... or they can come from stolen ones. Not that I agree with Apple's restrictions, but from a theft victim's point of view, would they like having the thief make good money from the phone?
That aftermarket car bumper? Sorry, can't use it, might come from a stolen vehicle.
Or some guy that figured it out with or without insider knowledge (which also usually means anyone can make a key for your car).
Specialized smaller shops also have the ability.
Currently there is NO WAY for me to change a screen on apple device without an annoying message or faceId issues on the 13. I cannot purchase a programming software.
Trivial solution for someone like apple.