What's interesting to me, as someone who is currently learning Web development mid-career, is that I can see that vanilla JS is actually a really good option nowadays, but not necessarily available to anyone who's more established. Because it doesn't seem to have gotten good until fairly recently, and, for the most part, everyone had already made the important technical decisions before that happened.
So, even if it's not necessary now, there's not necessarily any escape. The problem with complex tech stacks is that, for a variety of technical and social reasons, it's much easier to add to them than it is to take away. You can't necessarily incrementally roll back any of the bits of an existing React-based site; such an effort is liable to spiral into a complete rewrite. It's relatively easy, though, to incrementally add new things in order to paper over whatever's bugging you at the moment.
The social story is similar. Coming from a position of knowing very little, the effort for me to learn the vanilla JS way of doing things is roughly similar to the effort to learn the React way of doing things. But, if I were already invested in the React ecosystem, that wouldn't be the case. I wouldn't just have to learn new tooling, I'd also have to re-learn my entire way of thinking about how to architect a webpage. Realistically, you can't, all else being equal, justify a radical re-tooling in order to achieve an incremental benefit like that.