I'm in a non-US timezone, so all I get in most (software-related) IRC channels is crickets chirping. SO is like a searchable log of an IRC channel for when my question was asked and answered 4 days ago.
It's true that freenode is relatively quiet when most people in US time zones are asleep. But even then, it's rarely completely dead. In the larger channels, it's quite likely that someone will be awake and answer your questions even then.
But you're right about the searchability of SO. That is one of the strengths having a good, searchable archive of questions and answers. Forums, some mailing lists, and Usenet also share that strength, to some extent.
On the other hand, some IRC logs have been published, and google search results do sometimes happen upon such public logs.
You can also keep your own long-term logs, just by turning on logging in your IRC client, and leaving your client idling in the channels that interest you.
