I actually vastly prefer this behavior. It allows me to jump to (readable) source in library code easily in my editor, as well as experiment with different package versions without having to redownload, and (sort of) work offline too. I guess, I don’t really know what it would do otherwise. I think Rust requires you to have the complete library source code for everything you’re using regardless.
I suppose it could act like NPM, and keep a separate copy of every library for every single project on my system, but that’s even less efficient. Yes, I think NPM only downloads the “built” files (if the package uses a build system & is properly configured), but it’s still just minified JS source code most of the time.