The difference is that web components provide browser supports low level APIs which frameworks can use to make isolated components. Sure, you can get the same

The difference is that web components provide browser supports low level APIs which frameworks can use to make isolated components. Sure, you can get the same effect in React. But that only works if everyone on your page is playing by the same rules and using React. I can drop a web component into some legacy page which is full of old school jQuery and global CSS styles and my component will be isolated and work. Try the same trick with React in that situation and it will likely conflict terribly and get stomped by the old legacy code.