This may not be the whole story, but one of the reasons is that JavaScript does not have a standard library. Corollaries are: several different module systems, application frameworks and bundlers exist.
I don't know all the reasons for that, but I think part of it is that developers create them in order to put it on their resume. "My package is downloaded 500k times a week"
I've seen this same weird line parroted by so many people in the comments here: it's very odd.
If you're talking about server & browser having different APIs that's equivalent to any other language having different platform apis.