Because when you host it yourself, it can be off of the public internet.
I'm not saying it's safer to self-host. There are a ton of foot-guns with operating your own IRC server.
If your network and/or workstations are compromised, it is _over anyway_ because they have all your data. This is one of those situations where you are saying "What if they decapitated me? Slack might still be secure."
I mean, technically, you are correct but it isn't relevant because you are dead.
If you think such a business can survive a pentest from an employee workstation...XD