It's a fair question. If you're using a zipfile you can random-access the individual files in the zip easily enough. The node_modules folder is famously heavy with thousands of tiny files, so there's likely to be performance wins to be had. Old, less fashionable programming languages, like Java, of course already do all this with .jar files.

To make your point explicit:

tar archives don't support random access. So it would be slow to scan for each file in the archive as you load it.