That's a fair question... I've been building web based applications for over two decades now... I've been a fan of JS since before the "Good Parts" book, though that spelled out a lot of what I already knew. There are a
LOT of developers that use and know a little about JS.
However, the vast majority of "JavaScript Developers" don't know JavaScript all that well. Most haven't taken time to learn the newer features and/or syntax. Many try to write JavaScript like it's Jave or C#, or whatever else instead of using it for its' strengths. Not to mention performance implications.
Most of the time a lot of that doesn't matter. Most apps have enough performance to handle their load, and if they can scale to 4-5x the users with minimal relative extra cost, it never will. But when you do need those extra bits, it helps a lot.
I've worked on a few node/c# apps where each ms counts... getting an extra 10k requests/second on a large server, or reducing latency by 10ms is a win. Not all things are equal. One should evaluate each use case.
To each their own... I only dabbled in RoR and wasn't much into it. Ruby seems like a great next gen Perl imho, and I thought that was cool. I just didn't like the everything in the box RoR experience. I lived through the early ASP.Net and didn't want a repeat of that. I like the lego blocks approach that Node and .Net Core are taking. I try to avoid many of the bigger pieces/sets.