I'm not sure that grids have a role to play in single page web apps. But I'm convinced that React only has a role to play in SPA's (or self contained widgets).
Why? You can even use it for static site generation or purely server-rendered websites. You're probably thinking of jQuery.
Convinced by what? Many do use it outside of SPAs.
I make web apps most of the time, and I love React. React + Redux is my goto architecture and I don't think there's anything out there to rival it.
But I don't see any sense in using it to create a blog, or a promotional website, or any other kind of website that is essentially a web version of something that could also live in print.
You're right, many people do use it outside of SPA's, but I think it's a case of; If all you've got is a hammer, everything looks like a nail.