Why kernel development still uses email

Not the same thing. The submitter can't edit that unless you use the description of the PR which with the button shows up in the merge commit message unless you edit it. Look at this MR on Gitlab[1]. You can see that there were 3 discussions about the code and all of them are "resolved" right at the top. Toggle the resolved discussions and you can see that reviewers opened questions about the code and I marked them as "resolved" (without sending emails containing context-less "done" comments). An email is sent when *all* discussions are resolved so the reviewer can go back. You can also re-open discussions that have been resolved if it is deemed necessary.

Github has nothing like this (yet). I hope Github gets this feature and Gitlab gets the "batch review" feature.

[1]https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5968