Contributing via GitHub

Could it? absolutely, but it requires maintainer buy-in and currently many kernel maintainers consider the difficulty of contributing via the mailing-list-patch-email workflow, to be a desirable feature, not a bug.

I don't entirely disagree; making it easier to contribute will tend to increase the volume of crappy contributions faster than high-quality ones, especially these days. But the difficulty hurdle doesn't just keep people from sending patches, it also keeps people from reviewing / testing them; the number of eyes on the mailing lists is high, but the number of eyes on a web based forge is much higher.

I don't know if it works out to a net benefit in practice, but some subsystems/maintainers seem pretty happy with their forge-based or forge-capable (via gitgitgadget style bridges) workflows, so as usual it's not a technical problem, but a social one.