It does. There are gradle/Maven tasks to upload your artifacts to public Maven repos, and they're easy to set up so that Travis publishes them whenever you create a tag.

But this allows anybody to depend on something that somebody else wrote even if he forgot to publish (for whatever reason). It reuses the upstream github namespace instead of creating many artifact group ids when random people publish somebody's else package

Sure, which is why it is sometimes useful. But people who make Java libraries that are production quality, usually don't forget to publish, and if they do, they are quickly reminded to do so. I am not aware of any production quality Java library from the past five years that isn't on Central/jcenter etc.. Older ones aren't on GitHub, either. In fact, supporting sourceforge might be more useful than GitHub, for that reason.