That is the same if you use e.g. styled components with React, where every component is self-contained and doesn't leak the styles. The mechanism is different, but the end result is the same, and oh it's a game changer IMHO.
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.