Absolutely. Can you imagine the amount of completely useless and untested patches they would have to wade through if it was as simple as forking `torvalds/linux

Absolutely. Can you imagine the amount of completely useless and untested patches they would have to wade through if it was as simple as forking `torvalds/linux` on github and submitting a pull request? Maintainers would quickly learn to ignore patches from emails they don't recognize, and ultimately it would make it much harder for someone to break in. Sending the email to the correct list means directing it to the relevant subsystem maintainers. It's much more than just a patch submission method, it's a core part of their review process.