Why are these UI frameworks tied to React, rather than working for any HTML+CSS site (as with Bootstrap)? Do they provide interactive functionality that requires React or similar?
They are React component frameworks for common functionality with a unified style approach.
Something purpose-built for React is usually much better than something where React is bolted on to an existing framework, like Reactstrap (React + Boostrap) because those usually are built with jQuery and just handle state and functionality in a way that's not easy to integrate with React cleanly, so you end up re-implementing the actual functionality again.
The reason I asked it is that, if you're building an HTML+CSS framework for use in React, then why wouldn't you separate out the HTML+CSS part, and then depend on this. That way it would be easy to re-use 90% of the work, if you then want to use the components with Vue or some other framework, or even for plain static pages.