That optimization includes things like email filters and email client customization that is individualized to longtime contributors, not to mention that it is just what Linus and others are used to. And the long time contributors have had years, or decades to incrementally set up their tools, and become familiar with the workflow. The problem is that new contributors and maintainers don't have that, and learning the workflow, and setting up tools so that the email based workflow is manageable is daunting and takes a lot of time.
I won't contest that there are advantages to the linux Kernel's workflow, but there are downsides too, and a major one is that it scares off potential contributors.
That said GitHub definitely is far from perfect as well, and has different strengths and weaknesses from email based flows. As do any other options.
But just because there isn't currently anything that is unilaterally better doesn't mean things can't be improved. There is clearly a problem with onboarding new developers to the linux workflow. That should be acknowledged, and a solution sought. That solution doesn't have to be switching to GitHub or similar. Maybe there just needs to be better documentation on how to set up the necessary tools, that is oriented towards developers used to the Github process. Maybe there needs to be better tooling. Maybe the mailing lists need to be organized better, or have the mailing list automatically add metadata in a standard, machine-readable format to emails. Etc.