The argument presented in the article here is that libraries like jQuery and React were useful, but now the browser has better APIs, so we should use them.
The browser indeed has a components API, but it is by the admission of many, fairly low-level as far as component APIs go. It doesn't really solve how to manage data flow or rendering in your components or application, and has many sore spots.
No problem. There is a pretty nice library called LitElement that provides solutions to some of these problems.
But then we're not really living off the land are we? We still need Lit in this case.
So what's wrong with WebComponents? Frankly, I think this is a bad question. A better question is why people think WebComponents competes with React to begin with. WebComponents intentionally fails to solve some of the most annoying problems with writing components because it is intended to be low-level and used by libraries like Polymer. Because of this, it leaves many problems open-ended. Like for example. WebComponents act like HTML elements. Cool? So you can pass data down using HTML attributes. Neat. Problem: HTML attributes are strings. Okay... So you either need to serialize everything to strings, or you have to pass objects through a separate side channel (usually properties that you assign separately.) In fact, generally speaking, writing WebComponents that compose other WebComponents is ass. I'm pretty sure it has been noted before that WebComponents works best for leaf components... So it would certainly not make a good replacement for a library like React, which is meant to be a substrate you can build large scale applications out of.