Not sure if it matches your criteria of a mature component framework, but there's Foundation and UIkit. I've used both but all of my new projects use Tailwind. It matches my needs better because I use minimal JavaScript. It also doesn't get in my way and the documentation is refreshingly clear.

https://get.foundation/

https://getuikit.com/

> And why aren't there more?

It seems to me these tightly coupled CSS + JS component frameworks are in an awkward position these days. React/Vue allows you more flexibility. If a project is really simple you can use vanilla JavaScript or something like Alpine.js.

Perhaps the demand isn't there anymore.

There are lots, it's just that many mostly advertise themselves as component libraries for particular frameworks. Oftentimes you'll find that the common core is just plain ol' CSS with a little bit of JS in the background.

Not a framework, but a web component library I maintain: https://shoelace.style

There's Microsoft's Fluent UI: https://github.com/microsoft/fluentui

Not sure how mature, but here it is: https://niui.dev