Regulation. Having a full record of important conversations to fall back to for legal reasons is a big part of both corporate and governmental policies. Clinton's emails being deleted being an example of why ethereal messages are not taken lightly
IMO thats the magic of IRC. Channels i care about i bounce, channels i do not care i dont bounce in. If messages would wait for me that would mean so much overhead for the protocol.
I like IRC because its simple. I've build a IRC out of boredom, and a bouncer because the one i used missed a feature i wanted. Please nobody take that simplicity away :/
I just thought about the server implementation. This is simply not dueable. IRC servers are ment to run thousands of users, caching messages would be awful.
(Don't become an software architecture astronaut. IRC is also a terrible protocol. It was still successful)