Although I understand your point, and certainly have some friends that would feel like they're hacking the gibson connecting to an irc server, is Discord et. al really that much different?
It’s an inertia issue. I already have iMessage, FB Messenger, WhatsApp, and Signal as “primary” messaging apps, plus Instagram and LinkedIn and Teams as secondary— my tolerance would be basically zero for installing something else to connect with a specific person or friend group.
That is because you already have an absurd lots of them though.
I have whatsapp and conversations and that's it. I only use teams on my professional computer, you will never see me install "work" on my personal smartphone and nobody needs a linkedin app, the website is enough.
iMessage is needed for texting, Messenger for neighbours and FB Marketplace, WhatsApp for group chats, and Signal to have a non-meta option.
I agree about LinkedIn and will probably get rid of it; I mostly installed it as I was going to a conference and wanted the easy option for swapping contact details using camera codes.
Yes, they just download the app and enter the server name, username and pw. It's a private server and I manage the accounts so there is no registration step. And they only need to worry about any of this when they get a new phone.