That seems easy to you. That would be easy for me too and most likely 90% of the people on HackerNews.
But the average person doesn't have a "random Linux box" in their house. Most people don't even know what Linux is. Most people would be overwhelmed just looking for the terminal emulator on their computer, before they even typed a command into it.
Most people don't want to manage an always-on linux box for a voice server. Most people don't want to manage port-forwarding on their firewall/router. Most people don't have static IPs at their house and wouldn't know how to setup dynamic dns to solve the problem. Most people don't even know what DNS is.
MOST PEOPLE just want a program they can launch when they want to talk to their friends. That is why Discord has been successful.
I'm not saying that's good. I am just saying that its the way the world is.
It's no wonder so many projects and FOSS tools fail to gain large userbases when it seems that most developers seem to be living on another planet entirely.
IRC is pretty similar, but much more fragmented and not really very user-friendly.
its not a technical challenge, its a moral challenge. it means doing what is good for the users even if they don't really know it
if you have public IP or use stuff like hamachi (at least that's how we did it decade ago)
https://www.pcgamer.com/how-private-is-your-private-discord-...
Discord makes you the product. It's gratis in exchange for letting them spy on you. If you don't know why that's bad...