It's not really just convention. Git commit messages map directly to emails for workflows that don't use web UIs like GitHub, with the first line being the subject and the rest being the email body. That's how you mail a commit to someone else for them to review it or incorporate it into their local repo.

If you don't format your commit messages properly then they'll just be a sloppy mess in any email-based workflow (like what Linux uses).

And the first line is the only line that's displayed in contexts like `git log`, so it truly is the commit summary even if you aren't emailing commits.