This is going to sound mean but I don't really know how to phrase it more nicely. People building backends in js/ts are doing so because either they, or a critical mass of the people they expect to code in it, don't know any better backend languages.
I don't mean for this to be judge-y. People have different skillsets. A nodejs backend can be the right choice if you're a full-stack dev or a solo dev and you mainly know js and you get a lot of agility and correctness by using what you know instead of trying to use a "better" choice but one that you yourself are less likely to use correctly or quickly in your own use case.
It can also be the right choice if you're a big backend java/etc guru but you know you want to set this thing up but then just monitor/oversee frontend/fullstack devs being the ones who do incremental modifications to it.
But yeah - without additional outside constraints and everything else being equal, it doesn't make a lot of sense to use nodejs as opposed to basically anything more boring - be it java, rails, etc.
The thing that surprises me the most about node based backends is that they forego both the platform maturity and static-safety of java AND the ridiculous amount of batteries included into django and rails, just to be able to use javascript, of all things. What you could rails-g in a minute you could instead spend a day looking for an npm package that won't be abandoned in a month or spend eternity rolling your own everything and then maintaining it.