By default container images are not signed (there is notary, but it's not commonly used - maybe notary V2 will change that - and I think the signature changes depending on the registry it's hosted on anyway?) which make it inconvenient to mirror.

Now, why are we still producing new package formats without mandatory signatures (containers, npm, cargo, etc) is not really clear to me. I guess everyone must think "those old crazy Unix folks signing their Deb and Rpm must have had their crazy reasons, but we have no reason to do the same" :) a more cynical thinking would say "it makes it inconvenient to mirror things and easier to build a business from the central repository" :)