It’s just they need a bouncer (an always online irc server you run) for things like push notifications (and chat history on servers without the chat history extension).
Bouncers (and some IRC v3 extensions on newer servers) allow you to be logged in to the same account from different devices.
I’ve also had some fun writing a bridge from discord to irc for some personal discord servers (but it uses some account impersonation hijinks that make it impractical for other to use).
I'm also extending it with support for Discord/Matrix/etc-esque communities (in a 100% standard-client-compatible way).
I know that's not the fault of the protocol itself, but that's always one of the biggest hurdle for IRC adoption when everyone is used to just open apps and see the full history at all times.