Why? The whole thread is about how keeping it simple is supposedly better. Using react when you know react is easier than static html+ajax. It handles a lot of stuff for you and you can then use the incredibly rich react ecosystem for everything else (authentification, forms). Even a pretty basic web app usually requires those, so again, it's simpler to just use react and it adds basically no overhead for a react dev.
Plus, the big issues with react usually only manifest themselves in larger web apps, so it's basically "perfect" for simple stuff. You could make a functional react page in a single file in a few hours.
And they could also be react apps. What's your point? For a lot of people, a suitable react app is easier to make than the equivalent static HTML page with vanilla JS. The "it should be static HTML with a sprinkle of JS so the filesize is extra small!" guy is the one overengineering, here. The SPA's 20 customers are all using modern web browsers on modern first-world internet connections and don't care or notice that the page is not Stallman-approved.
Stallman-approved pages will happily welcome any and all users visiting your site. Modern SPAs maybe not so much due to the overengineering and mostly unnecessary features.
Plus, the big issues with react usually only manifest themselves in larger web apps, so it's basically "perfect" for simple stuff. You could make a functional react page in a single file in a few hours.