There are semi-standard XML formats that can be used to provide file position, severity and message, which could easily be produced from CI actions and would give devs and reviewers a great view of failing tests, compiler and linter warnings, ... With links to the files etc.
Instead we are still stuck with scanning text logs to figure out what failed.
I always assumed this is not implemented to eventually upsell something automated.
This is still a great feature though, which will probably prevent a large amount of bugs/vulnerabilities, assuming they can minimize false positives.
To give credit where it is due, I'd also note that most of Githubs new features since the acquisition were already present in Gitlab [1]. Github will be able to commit way more resources to make it polished, though.
[1] https://about.gitlab.com/stages-devops-lifecycle/secure/
Edit: apparently both Gitlab and Github have at least a limited version of this now, although Gitlabs implementation seems much nicer. See below.