Apple’s iMessage Encryption Has One Major Security Flaw
wired.com
wired.com
Uh because most people would have no clue what it meant/did and just be more confused. Don't get me wrong I'd live this feature but I'm in minority and even with it I'd probably only rarely check the keys against friends.
Just because something is possible (showing key fingerprint) doesn't make it "unclear" why it hasn't been implemented....
Though you could make the argument that by allowing manual key verification, Apple increases its trust with technical and privacy-conscious users. And while the vast majority of people might never know the feature exists, the technical users would become strong evangelists for the company's products.
It's a good long-term strategy for Apple to try to get the privacy and technically minded users on their side.