> Is one NPM package the equivalent of one object file?

No. The closest thing to a package (on almost every language) is an entire library.

> Can NPM packages call internal functions for their dependencies instead of relying so heavily on bringing in so many external ones?

Yes, they can. They just don't do it.

> Doesn't bringing in so many dependencies lead to a lot of dead code and much larger codebases then necessary?

There aren't many unecessary dependencies, because the number of direct dependencies on each package is reasonable (on the order of 10). And you don't get a lot of unecessary code because the point of tiny libraries is to only import what you need.

Dead code is not the problem, instead the JS mentality evolved that way to minimize dead code. The problem is that dead code is actually not that much of an issue, but dependency management is.