[hombre_fatal]

Probably because of how awkward it is. You essentially have this weird html-emitting "client-server" moving part in a limbo where it's still a one-to-many server. And it has to duplicate client code you probably still need anyways in the Javascript that runs in the browser (and, don't you want to do take advantage of some client benefits like being able to work offline or cache locally?). And this bonus moving part is only for your web clients, not something you use for any other client (iOS, Android, etc).

NextJS is something that folds over some of this, but it's not without its issues.

Of course, no better way to find out than try it for yourself. When I have the question of "why don't people just do X?", trying X myself is a quick way to realize why, and unfortunately it's never because I'm the first genius to have thought of it.