From what I can tell the reason is that that’s the way it was always done. As I understand it, back in the days before there were distributed version control systems, Bram was the only one who could commit to the code-base so he would credit the actual author of a submitted patch in the commit message. He continued this practice after the Vim project moved to Mercurial (and Git).

Related discussion: https://github.com/vim/vim/issues/1554

Ah, thanks

On reflection, it might have been clearer to have started my comment with:

“Taken at face value by most modern users of Git, the commit history does not accurately reflect the contributions to the code-base”.