Many IRC channels are archived and the archives are searchable, so the information is not necessarily lost.
Even if the archives are searchable (the searches generally suck in the first place), who's going to know to go into this specific archive, for this specific channel, and search for this particular query?
As someone who was interested in this a few years ago: This is still an open problem for someone to solve. The solution isn't particularly hard, it's just nobody has really gotten to it yet.
(And yes, you may come across the odd irc conversation in your google searches once in a while - but it is not to scale with the immense amount of information that comes out of irc).
Are you going to know that the problem you are encountering with your bluetooth connection was solved in freenode's #pulseaudio 3 years ago?
To me it helps create more of a community feel. Often times you'll see active community members turn problems they deal with on IRC into blogs, articles or even books to spread that knowledge.