There was a blog post here recently where a repair technician with his own shop was trying to contact Apple about reportedly stolen iphones being reactivated and resold and it undermining his business. They then found more and in one case it was a rogue Apple employee doing the activations, and in another it was software tools they found and send to Apple, which got ignored for more than half a year.
I can’t seem to find the original article but here’s Louis Rossman explaining the same thing. The argument boils down to the fact that Apple doesn’t care, more iphones means more people with wallets attached to them.
Also, it wouldn’t necessarily be public knowledge on exactly how it’s done, otherwise 1 Apple would try to fix the issue or 2 there’s money to be made in selling the service.
Edit: typo
What are you talking about ofcourse they can, it’s an activation which is stored on their servers, edit the FMI database and your phone is activated. They chose not to provide that service because they can’t know if your device is stolen and it’s way to much of a hassle to confirm you actually bought it. iCloud unlocking is a widespread phenomenon.