They actively reject ideas like package signing.When I was Django's release manager, I started our tradition of ensuring that every single package we put on PyPI was also accompanied by us publishing signed checksums of the file.
So. Django signs its packages. Now, what good does that do you? How do you know my key (or, these days, Tim's or Carlton's keys, since they roll the releases) is authorized to release Django?
"Just support package signing" is one of those things that sounds super easy. And in fact, PyPI technically supports it -- you can upload a detached signature along with your package!
But you don't "just" support signing. Signatures, absent a gigantic infrastructure of key management, indicating whose keys are trusted for what purposes and by whom, are basically useless. So when someone says "just support package signing", they don't really mean "just let us upload signatures!" What they really mean is "develop and maintain that web-of-trust infrastructure for me", but they don't like to acknowledge that's what the request really is.
Why are there no incidents where installing an apt-get package stole credentials
Because Debian grants package-releasing privileges only to a tiny group of people who are vetted before they get to release. Systems like npm and PyPI, by design, let anyone who wants to sign up and start publishing packages. That's a deliberate tradeoff, and one that comes with both risk (you'll get some bad actors) and reward (you'll get a larger and richer ecosystem of things being published).
I eagerly await your next set of soundbites that have come up, and been rebutted, in every single discussion of npm and PyPI that's come up on HN in the past five years.