People still use IRC. You can't run your own server and talk to people on other servers without a load of work, you cannot do any kind of encryption, file transfers (images?) are a privacy nightmare if they work at all, web clients need a proxy. Authentication doesn't really exist. Messages disappear if you disconnect and reconnect unless you set up an additional server that will make it seem like you're always connected.

The protocol was written for a different age. If you're going with old-and-proven tech, XMPP is a much better jumping-off point than IRC. Everything that made it simple and easy to use back in the day, makes it hard to use in a modern setting.

Once you add all that need on IRC its 2x as complex as matrix and harder to get into. There is a reason nobody does it.

Because people have different use cases and priorities about chat.

Matrix.org and liberachat used to be connected using a bridge, even then the Matrix side didn't implement the requirements of liberal chat fast enough such that the bridge was abondened.