It's not that hard to update a .gitignore file every now and then.
Using the actual tools built in to git directly removes steps in the process, which is always a good thing, it's documented as part of the git documentation, so you don't have to create a wiki page explaining why there is a ".gitkeep" file that git doesn't recognize itself.
Saying "It's not that hard..." is fine for projects with a few contributors but does not scale.
.gitkeep is explicitly not intended for git, because git doesn't recognize it at all.
Having the .gitignore, which is actually recognized by the git tools, means you can rely on the .gitignore functionality, including ensuring that things other than the .gitignore cannot be added to the repo.