I am reluctant to merge your PR if it has too many style issues or improper commit history. The last 10% can be important when maintaining a large project.
Because of the way git/github work, modifying someone else's PR (by pulling locally + modifying + resubmitting as a new PR) often means their authorship in the commit headers is erased, and I wouldn't want to do that without permission.
Edit: Not saying PR authors behave perfectly rationally either. Sometimes it's kids in long trousers failing to share their toys, and in the end "my ball my rules" wins.