What you can show/build with HTML/CSS/JS hasn't really changed that hugely in the last few years. There are RAD tools for React apparently (retool?) and I don't doubt they wouldn't be appropriate for all uses, but there's plenty of cases where you just need to throw together something quickly where being able to "build" the page layout visually would be extremely helpful. I sometimes get the impression many React-based sites you just get whatever layout happens to be the default (based on HTML5/CSS specs, and whatever libraries/out-of-the-box stylesheets you might use), even if the exact outcome doesn't really match what's been asked for by the UI/UX designer (who almost certainly did use a WYSIWYG tool to come up with the wireframes/static mocks etc).