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.
If you want to make a comparison you can equate things to hardware where things like quality control and yield is instrumental. If you buy a plastic case that after a year self-disintegrates because while it looked the same it wasn't manufactured to spec. When this could potentially happen to every part of your device this quickly becomes annoying.
Yet, this is how software is all the time and is further worsened by a lack of due diligence and quality control. How many developers can get the time to properly verify a technology or library. Not to speak of if it's even fair to expect developers to be able to reach those conclusions.
I do not buy that this is a problem. It's not an apples-to-apples comparison, but anyone can publish a 'book' on Amazon as easily as publishing an NPM module (and they do), however, no one is complaining that there are "too many low quality books". Why is it easy to ignore books but harder to ignore shitty projects? Similarly there are countless instances of bad poetry online, and very bad fan-fiction, but the sky isn't falling in the literary world. Can someone educate me: why do other people's shitty software projects elicit such strong reactions?