You just make whatever it is you want in HTML and make it reactive (put it in a file with the right extension and add the svelte-specific reactivity you want).
It's simple/straightforward, does exactly what you want, and instead of spending time mastering the API of some component, you mostly spend time using the HTML/JS/CSS you already know.
> You just make whatever it is you want in HTML and make it reactive
> ... instead of spending time mastering the API of some component,
If I were to achieve a reasonable amount of flexibility, I would have to redo the work of that library: create a flexible component from scratch with a good api. That is my concern at least.
So when I see the other frameworks libraries, I get the impression that possibly the target audience differ in needs, like so:
data-heavy, enterprise systems, business software => react
webshop etc, "simple" consumer oriented => svelte, solidjs etc