Because books and poetry aren't supported by package managers. They're self-contained, so you can ignore them and no one cares.

They also don't purport to solve a specific problem. (Like left-pad.)

And they're not boosted by a FOSS ideology which their existence a political choice and not just a practical issue.

So basically interdependence. You need at least some external code to finish most projects, and the chances are good it will be somewhere between overcomplicated, unreliable, poorly documented, poorly maintained, and unnecessary.

Maybe package managers need active best-of-breed curation and industry-wide coding and documentation standards. But professional standards are anathema to the move-fast-and-break-things crowd, so I doubt that will ever happen.