That's not the case at all if you go to a site that uses React. For those, you are welcome by a blank page with NoScript.
However, any React developer worth their salt (probably still a minority, as mentioned above) should be using ReactDOMServer which will render the page fine without browser JS being enabled.
In most cases though, you should be able to present a pretty useful static page by default without too much effort.
(Of course, as a Web user I consider both JS-only pages and SEO to be awful somewhat-parasitic practices, but that doesn't unfortunately doesn't factor into many business decisions :( )
Sometimes we see examples here on HN of blogs, that are mostly text content and pictures, built on a client side JS framework like React, but that's the exception.