Mumble is great, but there are some things that people have come to expect from group chat services that it is not designed to do. For example, running in a web browser, persistent message history, and multi-device access to a single account. Adding such things would be no small amount of work, which is probably the main rason it hasn’t been done. (And, of course, the changes required would make the result incompatible with Mumble.)

Considering what exists today, I think Matrix has the best chance at becoming a Discord alternative. It already has a lot of the needed features, and the new voice/video system (now in beta) looks very promising. And, of course, it can be supports self-hosting and end-to-end encryption, both of which Discord lacks.

Because no one wanted to?

You haven’t, right? Why? As you said, you don’t have the programming skills (neither do I).

It’s always just that simple - someone with the expertise doesn’t feel like doing it. Nothing more, nothing less.

Sometimes, people like you and me get motivated enough to go learn how to do these things, or find people with the skills.

It could just do with a UI refresh and maybe some added functionality

That is actually huge ask.
Mumble works in an “engineer brain” kinda way. Cause it has been made by engineers making sure the underlying tech is available to be used in so many scenarios.
Making it work in a “user” kinda way is a huge change.
And it would either make the code really difficult to maintain, or would isolate the power users by restricting the flexibility of mumble.
The fact that mumble is FOSS is absolutely fantastic!

Feel free to fork the project and refresh the UI.
Or sponsored programmers to do this. If there is actually a market, you would be able to overtake mumble. You can even start from their codebase, the license is very permissive (just make sure you credit mumble!)

Someone is creating a discord alternative: https://revolt.chat/