> component per-file React app

Nothing in React requires to have one component per file. And you can do the same with jQuery or vanilla JS.

> equally arbitrary CSS file

Something makes programmers, who are usually very opinionated about their code architecture & patterns, to just throw it all away and just pile on shitty css code. It's almost as if they don't actually put any thought into it. Could it be that their Java code patterns just cargo-culting and they don't truly understand it? That can't be it, something else?

> React is readable with HTML, JS, and CSS all right next to each other

React has JSX, not HTML. And nothing requires CSS to be in the same component or file. It's still just an organizational issue.