Most editors and word processors, along with other applications have undo and redo features that allow them to do that. If that could be tied with a timestamp, then it would be easy to see what version of the file they're working with. The only other feature they may find useful is a way to add a note to the file they saved indicating what changed (which would be the equivalent to a commit message).
Integrating those file(s) as a submodule or subtree in a git repository would work on the development side, but people who generate those files would have no need to interface with git, and people who use git would not need to use the editor for those files to get the history they need.
But I think the reason is because the mess of folders they then create in DropBox and they can never find their files.
What would be great is if someone could make a TimeMachine for non-developers that uses git, because that is what they are doing in their heads anywy.