I feel like this has happened because the people who are creating these solutions are used to working with excellent and experienced team mates, who have a solid understanding of JS and CSS and are happy to work with both (and waste time learning the latest build-systems etc).

I can see it getting verrry messy in wider circles - and I bet we'll go through a whole slew of bizarre process workarounds and frameworks until we end up with a solution that is suspiciously like "clear division between JS and CSS"!