Actually, people have. They just haven't packaged it all up into a happy-clicky single click deployment. There is a little bit of configuration involved, but I have confidence that a majority of folks here could do it in two cups of coffee or less.

For me, it is Convos (web front end that uses redis for state tracking) + UnrealIRCD (IRC daemon) + Anope Services (channels, users, permissions, etc). It isn't as pretty as the Slack UI, but it could be.