> Why isn't it the job of the distros to keep up on upstream security disclosures?

They can't, because (responsible) security disclosures are private, _not public_. That's the whole point of the system: notify the developers in private ahead of time (usually 30, 60 or 90 days) so they can write, test and roll-out the fixes before you release the info to the whole world. This is to minimize the time between when bad actors gain access to the exploits vs. when users install the patch. So "keeping up on security disclosures" cannot ever be a 'pull' process.

Usually the maintainers of the big distros are part of (private) security mailinglists and receive such info. Just not in this case it seems.