https://en.wikipedia.org/wiki/Progressive_enhancement
There were a lot of practical reasons for that: The browser landscape was much more diverse, different browsers had different support of standard Javascript, some browsers didn't even support JS and some people still kept text-only browsers like lynx/links in mind. Also browsers were not evergreen, so a large part of the audience could be on some older versions. Another thing were sometimes brittle network connection, especially over mobile. Depending on JS could in the case of corruption mean non-functioning websites.
For a lot whose exposure to web development and the discussions abound that, that reason will be stuck in their head, even if in the last decade of React ets the "best practices" will have changed.
There is also an aesthetic thing: There is a thing of beauty in simply curling an url and piping it into grep or such to get the thing you need, instead of having so have an headless browser. In my mind that is still how the web should work.
> Why the target audience of the ruby, probably primary web developers, whould do that?
In my experience, it's mostly web developers who care about this in the first place.
I’m not sure what you mean by this. We care about our users and how they use our websites. JavaScript is everywhere and has been the de facto frontend standard for the past few years. Supporting no-JS is starting to feel like supporting a new browser. As much as I’d like to, from a business and product point of view, the numbers are just too small for us to even consider it.
Maybe I'm not reading enough webdev forums. I agree though that things that don't required js should be written in no js way.