For some organizations unpredictable docker image releases are considered unreliable to use at all. Sometimes the image is updated the same day, sometimes after a few days, earlier even after more than a week.
The gap between developer tools and infrastructure support for them should be as small as possible this days. It's 2019.
You can always make your own base image with go installed that you update the instant you need to, if you really can't wait for the team responsible for the accepted docker image to do it...
I think that the question is, why wouldn't you? There are already a lot of work and testing done on projects that depend on 1.12 features, but using beta and rc releases. Final 1.12 version would just confirm that everything works, but without docker image, testing on staging infrastructure is delayed even the final version is out.
This is why I have https://NewReleases.io configured to track both Go GitHub releases as well as golang image tags on Docker Hub.
So you do not need it as you do not want to use it too soon. That is cool. Others may need docker image release aligned with regular release, and I think that there is nothing wrong with that. Especially for the people that are trying to contribute to Go by testing beta and rc releases.
The gap between developer tools and infrastructure support for them should be as small as possible this days. It's 2019.