They don't exclusively render server-side because it's tremendously CPU-heavy: https://twitter.com/kadirahq/status/620467416749838336

Over the years, Rails has evolved a lot of ways to do server-side rendering while sharing as much as possible between runs. React isn't there yet, but there's no reason that it can't do the same given enough time.

Until then, in many ways, as a user of a site with a modern JS-enabled browser, I'm providing my own CPU power to render dynamic content, thereby subsidizing the site's cost of supporting older/non-JS browsers with SSR at negligible cost to myself (though I suppose everyone's utility function handles occasional flickering differently). I, for one, don't see any problem with this arrangement.