>just

Why is building, distributing, and cutting over to a new container easier than "sudo apt-get upgrade postgres"?

Nobody said it's easier. But it's definitely more decoupled with a more well defined and reproducable binary image state which you can spin up and shut down in large scale easier. Nobody is saying that this is something you, your Postgres or most people need. Over 10+ million Docker hub pulls of the Postgres images saying there is a use case of using Postgres Docker images.