Doesn't an IRC server have no concept of chat history? Not really comparable. Setting up the server is the easy part, it's migrating their integrations, updating docs, copying over history, educating users, etc, that is the hard part.
The sibling comment talks about IRCv3 features that show a limited amount of context from immediately before you joined, which is not the same as the infinite history seen in Slack and Discord.
It takes additional effort and overhead to then go capture the pertinent details that exchange and write them in a wiki. With infinite Slack history you get a janky high noise knowledge base for "free". Huge downsides of course, but clearly it's a very popular approach.