1. Using the same test allows them to normalize your "skill level"[0] against the rest of the employees and/or candidates.
2. HR may be using test as a screening method before they loop in another developer to do an assessment. HR typically won't be able to evaluate your Github projects.
3. Your projects may not demonstrate skill in the specific areas they care about.
4. Your projects might be niche and not easy to evaluate directly by someone not familiar with the problem you are solving.
That said I find that if a candidate is in that rare group of people with meaningful github projects that I get a lot more value from just asking them questions about one of the projects rather than asking a whiteboard or coding test question in an interview. Most candidates do not actually have a github profile with meaningful projects that are not forks in them.
[0]: I use quotes here because I'm dubious of any claims that a coding test can give clear signals for truly useful skills in software engineering.
Lots of developers don't have public projects on GitHub because their work is proprietary and private. I can't show any of my code without getting permission from my customer because they own the work. A lot of public GitHub projects can communicate "newbie" or "hobbyist" just as much as it might communicate "experienced" because actual work product you got paid to write probably can't get published for the world to see on GitHub.
