Because, from this outsider's perspective, it seems like most of the tooling is library management (rather than, say, profiling or linting -- does javascript even have a linter? -- or debugging).
There are tons of tools, of course also including jslint, debuggers (e.g. IntelliJ, dunno about others). There are editors and even IDEs. Then there’s the problem of bringing (NPM) modules to the browser using tools like webpack or SystemJS or whatever. There are general purpose build/automation systems like gulp. There are transpilers that “compile” ES6 to ES5. For any given task, you can probably find at least five programs that do it.
And if you look at all the topics, you’ll find that library management makes up only a small fraction of everything.
Yes, thousands of them, npm alone has 1200 results for linter:
Yes, as libraries. There are multiple linter options that can be installed with a project. Debugging is available, usually through an IDE or browser.
There are tons of libraries for C, many of which aim to do the same, only faster/smaller/scalable/whatever. There are many compilers, both open and closed source. And there are documentation tools, syntax checkers, unit testing frameworks and whatnot.
Of course there are also languages like C# where there’s a de-facto standard for almost everything: IDE, compiler, base library, GUI framework(s).
I think choice is great!