Can't speak for anyone else... but will say I'm far more productive with full-stack JS. My second most familiar language is C#, followed by Ruby, VB, Java and several other languages. Most of the productivity outside the browser is because of Node + NPM though. Also the browser gives you a much more rich rendering target than pretty much any common UI platform out there. Including some very nice abstractions.

In the end, it isn't the best choice for many things, but it's good enough for most things. As such, it's what I tend to reach for first, since getting something working takes less time, and gets in my way so much less. Windows, Mac, Linux via Node, Electron, Carlo (node + installed chrome), not to mention Cordova (iOS/Android) and React Native. Best of all, I don't have to deal with a lot of issues seen in other platform specific tools.

It's so much closer to write once, run everywhere with less friction than anything else that has been done in computing. Aside from trivial projects.

I hear a lot of complaining like this from Java devs. Oddly, they are often okay only knowing one language (Java). The exact words from a Java dev were, "well, when all you have is a hammer, everything looks like a nail".