This is a very interesting point, because my experience is quite the opposite.
Most of the time changes to the apps that I typically worked on (ERP/CRM platforms) involved both DB migrations, as well as back end and front end work. And with the front end, changes to the logic almost always resulted in changes to the template, even if only within a single component. The only exceptions to this were validation changes (changed logic only) or visual bugs (changed template only).
I wonder what qualities of the system design this points towards, apart from coupling.
But for why I go so long without touching the UI code, it's just because the project is a 3D voxel game and there's a lot of big complicated parts to work on that don't involve any HTML UI.
https://vuejs.org/api/sfc-script-setup.html
Re: magic in Svelte, I haven't used it but that sounds like something I'd be wary of. I like how Vue 3 takes a functional and explicit approach to defining reactivity. It's no longer grouped into buckets ala Vue 2 (data vs computed vs methods etc) but rather you write normal non-reactive JS by default and wrap values in reactive() or computed() when you need reactivity.
The only syntactic downside is reactive values always need to use foo.value to access foo's data which causes occasional bugs when you forget it (and it tends to fail quietly) but it makes sense why it is that way. Maybe better LSP/IDE integration can detect that.
It's good to hear Svelte has .svelte files like .vue SFC. That would be the biggest thing I'd miss with React.
The reason this is huge is because 90% of my coding is not Svelte. Usually I'm working on app logic, so I might go weeks or occasionally months without touching any UI code. With Vue I used to dread making UI changes, because I always wound up needing to re-learn which properties I needed to put in which return value to which callback (or somesuch, I don't remember the details). I'm sure it's very intuitive if one uses it every day, but it never was for me after a six week gap.
In contrast with Svelte I feel comfortable doing UI work after a long hiatus, even with nontrivially reactive bits, because apart from the templating it's basically just plain JS (or looks like it). It took a bit of bother initially architecting stuff, but I don't think I've checked its docs since I first switched to it.
That said, the big tradeoff is that Svelte's reactivity is fairly magical. Personally this doesn't bother me, because as a JS main I find it easy to intuit what Svelte must be doing behind the scenes in order to work the way it does. But if you're an occasional JS user you might hate Svelte's magicalness for the same reason I disliked Vue - e.g. you might keep needing to re-learn when to use $: statements, etc.