That is because it is NOT written in JavaScript.
I have written JS since 1997 and kept up with ES6 and so on over the years. Used it in frontend until now.
The JavaScript-ecosystem just isn't very productive [1] for multiple reasons. The language is untyped. You have no multi-threading. It's hard to debug. You have to spend many brain-cycles on learning Browser-problems, which muddles your understanding of the language. The web-way of: we can fix it and everyone will reload the browser, leads to a "don't care" attitude.
For web/backend this is approachable, though still not good.
For build-systems this is terrible. Why? Because if a bug shows up in a build-system you stop a million developers from being productive. The bug won't get fixed right away, most will just work around it, use different plugins, copy/paste yet another webpack config etc.
Plus JavaScript tends makes people realized what FP is, and they all start putting functions in their functions and making everything modular and plugin-able. (See high-order-components in React). While this can be nice, it leads to even more confusing bugs.
Writing plugin-architectures is hard in any language. Writing it in JavaScript is a recipe for disaster.
Hence the modern JS-ecosystem.
[1]
Google: nullpointerexception -> 4.360.000 results
Google: undefined is not a function -> 800.000.000 results