this is a very interesting question and I think it boils down to the philosophy of the project compared to Elm/ClojureScript. The latter are meant to be battery included, fully integrated, while ReScript is seen as a drop-in replacement of JS, a compiler that generates regular JS and that is meant to Just Work TM with the rest of the JS ecosystem. So if you’re trying to do something with ReScript, you’re basically supposed to try to make it work with JS and then just bring ReScript and you should be good to go.
There’s actually a manual setup guide if you don’t like create-rescript-app.
I understand this can be an issue for newcomers in the web ecosystem because you then have to learn two things, the web and ReScript while you can just learn Elm/ClojureScript. But it also makes gradual migration and reusing existing JS tooling much easier.