That's usually because those tutorials are also setting up sever side rendering. Node.js is only needed for that and for running the various (optional) build tools.
When doing a SPA I often set up Node.js to serve the index and provide server-side rendering and then never touch it again. Any actual API/WebSockets/RPC interfaces the app needs can be serviced by any plain old Ruby, Python, PHP, etc app backend. Since I usually use PHP from old habits, I use a Symfony API-only application.
I posted about this in a top level comment, but don't forget React can control any element in your DOM and not just the entire body like most SPA's are setup. You can continue using whatever languages/frameworks templating and view systems and augment views as needed that would be better handled by React.