How would you compare candidates that have some code in GitHub and those that don't ?

You can assess the ones that don't have code on GitHub in a different way, but there is no reason to ignore a possibly valuable datapoint.

But faang companies try to objectively compare all candidates. Also they have so many resumes that it doesn't seem feasible to review code on GitHub.

5 hours x 1/2 people checking boxes is energy that could be put to better use.