JavaScript is also not the language people write in, it's the target. People compile there code into JavaScript more so that writing it.
The reason you have JavaScript on the server side of things is that it bridges the browser gap. You get to share some bits between your client and server code. Also, if you have hybrid rendering solutions it's really convenient.
You don't exclusively run Node everywhere just because you have Node.
It's a tool that can do some things really well. That's it.