I agree that RedHat and Debian use OpenPGP mostly for its signature only and do not rely on any Web of Trust or other infrastructure for key distribution in the basic case. As a side-note just for fun:
It's interesting to consider third parties who maintain Linux package repos, like Postgres https://ftp.postgresql.org/pub/repos/yum/
They distribute their public keys on their own web infrastructure, so it's effectively TOFU. In your criticism of PyPI, you rightly pointed out that keys are useless unless pushed to a publicly-known key server because they aren't discoverable for users, which leads to ~no one verifying installs. I agree with this for PyPI, but in Postgres' case, if you do not have their key pulled locally, installs will fail by default. This means that users do generally install keys locally (actually they typically install the repo RPMs which configures local repositories as well as installs keys), even if the discovery mechanism isn't enforced by yum. My point here is that ecosystems without the ability to pre-bake rings of trust can still result in ~all downloads being verified as long as the client tooling defaults to the correct thing. pip not doing so is the main reason why OpenPGP signing never caught on in the Python ecosystem.
But yes, I agree: A modern day dpkg and rpm could swap to something with safer cryptography rather easily without changing anything about distribution. OpenPGP is a lot of bloat for their needs.
Out of curiosity: which government standards? I'm not aware of a US Gov't standard that mandates PGP; this would be very valuable to know.
I don't believe any mandate OpenPGP specifically. I was more trying to get across that many institutions mandate FIPS 140-2/3, and current implementations (like on RHEL) complain if signatures are not present in their expected format. Because OpenPGP is my only choice for signing yum infrastructure and RPMs, it's a de-facto government standard today.