Because you can't practically use web components. Only Chrome and Opera support them. Angular2 has support down to IE 9.
Which is why I stopped messing around with polymer. Basically adding the webcomponents polyfill to your DOM tree (because it touches so many things) breaks contenteditable in some weird ways. Which can be limiting to people who use rich text editors (many still decorate contenteditable divs) although iframe-wrapping the editor fixes the contenteditable breakage (at the expense of introducing other problems with extending your editor - try adding custom (say) atmention autocompletes to something in its own execution context).
Also, webcomponents have had some pretty major syntax changes over time, especially when it comes to shadow-piercing styling vs other styling selectors.
Ultimately, I'm looking forward to webcomponents. I can't wait to have proper namespaces and all that. In the short run, though, I've tried the webcomponents polyfill (I wrote a lot of polymer 0.X code for a webapp that I eventually ported to angular 1.x instead when the polymer 1.x syntax meant I'd have to rewrite a lot anyway), and it's not Enough.
React is great as well, but if you find Angular incomprehensible it's not clear you'll find React much easier.
One major difference between React and Angular is that the Angular community _tends_ to separate their templates from their "controller" code, whereas the React community _tends_ to intermix their templates with code ("PHP" style).
The result is that in Angular you have a few funny characters in your HTML to learn (e.g. `*ngFor="let foo in bar"` for looping) whereas in React you'll use JSX and use JavaScript for loops.
It's pretty clear to me that component-based webapps are here for the long haul. Even if WebComponents ends up being the future, the concepts you learn in using either Angular or React will be more-or-less transferrable.
It seems to me that Angular and React are becoming more similar over time. For typing, Angular uses TypeScript and React uses Flow. For data architecture, both communities are gravitating towards some sort of Flux (redux / ngrx). You can even use JSX in Angular if you want to.
Shameless plug: I've written a book on Angular 2 which talks through, step-by-step, how to understand Angular. The first chapter walks you through your first app and you can get it (for free) here: https://www.ng-book.com/2/