Yes and no. It's not 'hard' it's just friction that's not present at baseline (when iMessage was designed, and for the most part still today). Sometimes you send messages to someone who you haven't ever met in person, or you met but didn't think to exchange keys at the time. And when Dave drops his phone in the lake and buys a new one, he would have to sheepishly re-meetup with everyone he interacts with.

If iMessage had been designed to require a brokerless key exchange, its security would be superior (though in this case, Apple's interception software trumps everything). But iMessage would appear to be less convenient than alternatives like WhatsApp (brokered key exchange, re-keying).