But this can't be the whole story. In the Java world, it's pretty common to import a couple huge libraries full of utility functions, but those are each one import, that you can track and version and pay attention to.

Apache Commons helper libraries don't import sub libraries for every little thing, they collect a large toolbox into a single library/jar.

Why instead do people in the javascript ecosystem insist on separating every function into it's own library that STILL has to import helper libraries? Why do they insist on making imports fractally complex for zero gain?

Bundle size optimisation. See my comment upthread for more detailed explanation. Bundle size is one of the historical factors that makes JS ecosystem a unique culture, and I'd argue uniquely paranoid.

I didn't used to be. It's just become less trendy to import a big giant Lodash, Underscore, Sugar, or even jQuery.

Originally I think it was to avoid the applet experience of downloading a large util.jar or etc. (Not that most js devs really care.) However, I suspect the motivation is often social status on GitHub & their resume.