Web Components only really covers half the picture here. Libraries like this provide integration with their chosen framework, including tying events into however the framework should do such things, and places where data flows without DOM nodes (e.g. if a widget renders based on some dynamic data source that isn’t child elements). Often you could do this with Web Components, but it would substantially harm expressivity and/or performance.
Also you can generally make the implementation lighter and faster if you implement it within the framework, especially if the way you make Web Components uses a different framework (so now you have n+1 frameworks on your page).
Web Components can be barebones, use no dependencies, do all DOM manipulations directly. That would give you vanillajs performance. It won't be easy to scale to big components this way--the use case is small components with minimal state, something like a time picker at most. Integrating the Web Component into React should not be much harder than integrating an <video> element. At best, do nothing or at worst, create a Ref and attach an event listener. The benefit is this would work across all other JS frameworks. Web Components are primitive, like the C ABI, but everyone can speak it, like the C ABI.
Also you can generally make the implementation lighter and faster if you implement it within the framework, especially if the way you make Web Components uses a different framework (so now you have n+1 frameworks on your page).