I wonder if it's because it opens in the default visual editor (typically vi/vim) and while I'm primarily a (neo)vim user now, it is certainly not the most intuitive editor to get started with without guidance.
git config core.editor "vim"
A lot of talented developers that I've worked with use visual git clients exclusively, so imagine their horror when presented not only with an editor they don't use, but also a text-based menu (`git rebase -i`).