Key exchange would be easy in real life, just bump phones when you meet someone. It's only difficult on IRC.
We are over a year into a pandemic which involved wide-scale lockdowns. Physical key exchange is a nonstarter for broad adoption.
I think the emphasis is on the -ed in involved. That's a temporary condition which is already resolving in much of the world. You may also be overestimating lockdown compliance among average people.
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).