It's not "very simple" - it's just that with Slack, someone else is running ZNC for you.

Chat apps don't magically "just work" - either you run your own infrastructure, or use someone else's.

Even with IRC you are still using someone else' infrastructure -- unless you run your own IRC network from scratch of course.

If you ever want to feel like you're bashing your head against a brick wall over and over, set up an IRC server with channel/nick services.

WeeChat + relay clients can do it as well, you only run weechat in a tmux/screen somwhere and connect to it with a relay client (Glowing Bear or WeeChat-Android). Works nicely, with synced state and arbitrary history scrollback