In theory it is completely an implementation detail, I agree. In practice, it's bloat. If every web component I use in my project might pull in a completely different framework behind the scenes (and worse: if those web components depend transitively on other web components that pull in yet more frameworks), then I now have to deal with all of those different frameworks. Each of them will load more bytes and make my page's startup time slower. Each of them will have their own state management systems. Each of them will have their own approach to templating. Each of them will behave subtly differently in practice.

Why bother when I can just write everything in a single framework?