Packages are typically different once published than they were inside their original repositories. Call it transpilation, build, compilation, packaging, etc, most popular projects require some level of support for dynamic code execution before reaching their usable state.

As much as I'd have liked Git to be a viable option compared to centralized registries, last couple of years demonstrated running arbitrary commands during install is too much of a risk for it to work at scale.

   ...most popular projects require some level of support for dynamic code execution before reaching their usable state.