It is used because UI component frameworks make vanilla CSS an awkward an unnecessary tool (the Cascading part). CSS makes sense as designed for styling a document, but not so much for styling atomic components. If we were designing the web and CSS from scratch around the React model, CSS would not be made to cascade, because that is totally unnecessary and usually leads to more issues than it solves. Apart from theme variables, in a component system there's no reason to have global CSS anywhere; you'd just style each component in isolation. So when in the React model you typically want no cascades at all, the library that is made for that gets popular.

Once you get rid of cascading, and you have one stylesheet per component, it is again unnecessary to have the one stylesheet in a different file from the one component. There's no benefit to splitting a component and its styles into two files; it just makes it more annoying to edit. So we move the styles essentially inline, with either Tailwind or CSS-in-JS.