It's because submitting a form interferes with all the other stuff running on the same page. E.g. after submission animations stop or disappear or skip a frame, annoying designers, etc. etc.
It's not always the right tool but in more cases than modern devs believe, it is.
I've built several web applications recently that don't use any modern JavaScript framework. You'd be surprised how quickly a page can fully reload and equally surprised at how okay of a UX it is to not always persist state.
I don't think there is really anything in your "etc. etc." I have never heard from any real user a complaint about animations stopping mid-flow. I think people know how pages work, in that I think they expect clicking things to do something disruptive, altering, changing. It's only been the design astronauts that have argued these discontinuities are "confusing" to users.
If anything, sometimes I think they make the transition too smooth, to the point that it can be possible to miss it. Sharp changes are generally easy to notice.
The sites that genuinely need a react frontend are fewer than the sites that use a react frontend. If you forgo react from the start, you have simplicity. At most, track your "state" in a session cookie + "rendering" on the backend. The good old days. I miss them.
