It may be the default today, but it largely earned that position by being one of the better options out there. Today there's alternatives and even Angular still has a decent following, not that I'll touch it if I can avoid it.

edit: Just adding to the pain at the time... iirc Webpack + Babel + Sass + CSS + ReactTransforms each with wierd bespoke configuration options... Babel itself was a massive pain for even trying to limit to modern-ish targets or multi-target.

React itself was a bit awkward as well, a lot of the concepts themselves were difficult, and IMO, it didn't get much easier until functional components, even if that really complicated the library itself.

I still have mixed to poor feelings on Server Components as I think it's largely a waste for the types of things people typically build. HTMLX (speaking of innovation) is likely a better option in that space.

That said, I do like MUI (formerly Material-UI, a Material Design Implementation), I think the component architecture is really thoughtful and works well, biggest issue is that devs don't take the couple hours to read the docs and even have awareness of what's in that box.

I also like Redux and even hand-written reducers and extensions quite a bit.