I think image is the important thing. The one time I bought a used iPhone from a shop (reputable one), I received a device sold as "refurbished as new" with a clearly subpar third party display (protruding, wrong colors). Had I not known how the phone is supposed to look like I wouldn't have a great opinion of Apple's build quality.

Now, Apple does have authorized and official repair shops. Clearly they don't have enough of them and they treat them quite poorly too. When I called one that I needed replacement AirPods Pro (already with a ticket from apple), they told me the wait time is 3 weeks because they can only source 1 part per week (and the two earpieces and the box are counted separately).

Apple is fine if you live in a country with Apple stores, but outside of that it's a big risk.

And it can avoid bad actors replacing parts in your phone to spy on you but I don't think it's Apple's main motivation.