These are devs who would rather build tools to build a web app than to build a web app. Different people have different itches. We don't have to consume what they produce. Just have to read about it on HN.
Well and fine, but those tools then need to be maintained forever, and they become yet another tool in the stack for developers who come on board to a company which has adopted them. It's easier and quicker to learn a new library like React, or even a new DSL like JSX, than a new language.