Best guess, a reflexive need to keep diffs as small as possible. Personally I think this is a completely wrong mindset, having version control is what allows you to go wild because you can always use the version from before a crazy refactor - and if it goes wrong you can even keep it around on a branch for reference later on with a second attempt.
You don't have to look at the history to use other features of version control. Typically everything is moving forwards in a repository.