Separation of concern or technology? A well structured React application is much easier to understand than something where you need to keep 3 files in your mind at once.
Or worse when those three files are in three different, mirrored directories.
It used to be a separation of conterns. HTML for structure, CSS for presentation, JS for some bells and whistles.
Now, when everyone forgot what the web was about and see it just and app platform that does not really apply.