It's extremely straightforward: Apple is in the user experience business. People buy Apple products because they 'just work'. That reputation is worth hundreds of billions of dollars because most people have frustrating experiences with tech.

Unreliable devices damage Apple's hard-earned reputation. An end user won't know why the Apple device they're holding is unreliable - they may not even know it has been repaired if they acquired it from someone else. They'll just know it's an Apple device, and thus it must be Apple's fault. When that person talks to someone a day or a month down the line about their experience, they will not have as good things to say. In sum, this reputation will depress sales and customer satisfaction, which are the two key metrics for how Apple measures success.

Building reliable tech that just works is hard enough. Non-OEM components and third party repair shops add additional business risk, which is why Apple takes a pretty restrictive stance.