Because it is too complex to justify maintaining as the free self-hosting solution and too complex to support for the average inexperienced admin?
But if you follow that guide perhaps that approach still works.
Can’t comment on CDCK priorities/roadmap
I do find it (that is “docker manager” updates) unreliable: I often find if there is an error during an “online” build, I’m unable to retry the online update after fixing the issue (usually with a plugin). The online update process appears not to respond, so I’m forced to perform a command line update. I’d raise a Support Topic if you come across an example when that happens.
Not with the basic install because that’s rebuilding a single container and because that potentially requires a new image and potentially affects the database, the whole site has to be taken offline.
There is a two container solution you could try which allows you to rebuild the web container separately that may give you more uptime: