They are not CSS frameworks in the sense of Bootstrap.

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.

Probably because the people making them are using React, and first and foremost they're making it for themselves.

That was my guess, but the_duke's comment makes me think there is more to it. I'll have to look at the code to see if the React components do anything more than apply HTML templates with associated CSS styles.

Some of them require run-time JavaScript, like the popovers and select boxes. And largely the value here is in the comprehensive component API, rather than just the HTML/CSS it spits out.

Not sure about interactive functionality. But it basically means it provides ready-made React components.

I think the_duke answered my question.

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.

Bootstrap needs jQuery for anything that requires interactivity