"Critical" projects and volunteer maintainers

In addition to the things others have mentioned, the suggestion that we should only have system package managers and those should "curate" the libraries has a big bootstrapping problem.

Distros are limited in the amount of new packages they can review. The queue for new packages in most distros is hundreds to thousands of packages long. As such, they usually require that a package be of a certain age and popularity or depended upon by something that is.

So yes, you can say, "why would I use pypi/cpan/etc. when all of the best libraries are in my distro already". But that's ignoring that the only reason that rich ecosystem of libraries exists the first place is because enough people were willing to install it in other ways that it became worth packaging. And that popularity was only possible because installing it those ways was easy for users[1].

[1] It's also worth considering that the only reason your distro can afford to maintain those tens of thousands of python/perl/etc packages in the first place is because those packages exist on an index, in a format standardized enough that you can just use automated tools to repackage it with little extra work, instead of as disorganized tarballs on Some Guy's personal website.