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.
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.
Here's an example video of an app that does a mix of "state persistence" and "good ol' reload the page" approaches. https://withlattice.com/documentation#nav-create-application
1) Chat interface maintains state (but uses minimal JavaScript)
2) Application status requires a reload
Is this the absolute best UX? Possibly not, but users haven't said anything about it and it's such little code with almost zero dependencies.
I've found that it's only like 1/1000 that will.
And this thread (few days ago) about why it's hard to buy nice things touches it https://news.ycombinator.com/item?id=42430450
Many (most?) don't raise issues because they've been conditioned that things won't get fixed.
I'm with you that overdone JS isn't necessary; and we should also consider user-silence to issues to bloat, slowness and jank.
I love a beautiful UI and experience. Listen to users and prioritize what helps them.
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.
"etc. etc." = anything with state, so this can be quite a lot actually ...
Or worst case scenario, fetch().
But honestly, forms are fine. Not all apps need beautiful animations.
It's better to have great UI, but it's not free.