Angular directives, Ember components, or any other framework's components are not the same as Web Components. The browser doesn't, and won't ever, understand what they are. They have very weak encapsulation unless used solely within a single framework. You can't easily use them across frameworks.

The current battle of frameworks has lead to massive fragmentation. Pulling together a React view with an Angular dropdown and an Ember tree view is damn near impossible.

Web Components are real elements. You can use them wherever you can use elements - including in all those other frameworks. That's the difference.