In practice "better UI" would mean things like being able to trivially share files and images, or quote/link a specific message, or even making it easier to distinguish between users with similar nicks via their profile pictures. And those UI improvements are actually features which are integral to its protocol, so they can't easily be bolted on by a custom IRC client in a backwards-compatible way.
Literally every single modern chat platform has support for stuff like that, and for a reason. Discord became popular because it combined those modern chat features with the ability for every community to create its own private little "server" - while at the same time making it trivial to participate in multiple "servers" at once.
Quoting/linking is a client feature, not a server one.
IRC servers do also support profiles.
I think the real “issue” with IRC is that its users generally prefer the minimal UI. So there isn’t an high enough demand to make prettier UIs. But there are web clients that are a little less basic.
For what it’s worth, I’m in that minimal camp too. I wish I could still connect Slack to IRC.