Some might say it's Stockholm syndrome, but then there's the Python developers doing the same thing everywhere. And the LISP enthusiasts. So unless there's a lot of Stockholm syndrome (although it's not out of the question when it comes to LISP), a lot of it is easily explained by "me like language here, me go put language elsewhere to keep language"
The easiest way to turn front end developers into full stack developers was to let them use the same language for everything, and it was easier to bring JavaScript to the server than to bring any other language to the web.
These days with WASM and JavaScript as compilation targets it might have been avoided, but it's already too late.
Anyway I rambled a bit but the point is "joy" has nothing to do with it.
Any decent developer can become competent with a new language in a matter of weeks, whereas making a large application with the wrong choice of language and tooling can become a maintenance nightmare. Given how difficult it is to verify JavaScript programs as "correct", its use on the server has always riled me up.
I'm only going to address NodeJS because I think that niche projects that target niche workloads are just unrelated.
I believe it does explain NodeJS. The idea is simple - your frontend engineers can own your edge services/ APIs. They're already heavily invested in JS and, at least at one point, there was a theory that unifying languages across frontend + edge made sense for other reasons.
Some people really just learn one language.
I think that's a gross oversimplification. My colleagues know JavaScript, TypeScript and ECMAScript, as well as YAML, HTML and CSS.