Just because "Every React codebase I've ever worked in has used JSX" doesn't mean "you still need to transpile JSX in most scenarios". Of course, if you're serious about using React, doing the transpilation before pushing your code to the CDN probably makes the most sense, rather than doing the transpilation client-side or using vanilla JS. But just because it's common, doesn't mean you need to follow that way of doing things.
Same with React. There is a production CDN https://reactjs.org/docs/cdn-links.html