Also, it's a sort of "legacy" feature. Earlier versions of the PR UX made it tough to review commit at a time instead of just the Full PR Diff Firehose. (Now you can pull changes apart into commits or "updates" and several things between directly in the PR interface.) Commit comments and those automatically floating into relevant PRs was basically the old UX flow workaround to that. It was slightly less of a niche feature then. Now the PR UX supports those use cases more directly and there's much fewer reasons to use commit comments.
