Some parts need to be cryptographically linked for security reasons. For example, when the phone used to have a fingerprint sensor I believe all communication between that module and the rest of the phone used public key cryptography so that you couldn't replace the sensor with something less secure.

A second reason for doing this is to make theft of devices less lucrative. If you can't sell a stolen phone for parts, then there's less incentive to steal the phone.

I think there's more that Apple could do to make third party repairs easier without sacrificing security or theft disincentives. Maybe there could be a setting where I, as the owner, can authorize parts salvage of the phone before I sell it.