That came partially out of the desire to reduce the lure for thieves and robbers. It was really bad during the first generations that regularly had jailbreaks and ways to bypass "Find My..." or whatever, then the first tightening reduced resale values of stolen iPhones by a good amount (as they were only good enough to slaughter for parts once reported stolen), and the latest round made it even worse for criminals.
Personally though, I'd preferred they simply provided "unlock codes" with a phone that could be used to remove the association between a part's SN and the IMEI/SN of the phone. That way, buyers of iPhone have something similar to a certificate of authenticity.