We get this one a lot. There are many possible answers, and for some, multiple answers might apply.

• The editor doesn’t want to. It can be as simple as that.
• The editor realizes that there is more money in editing than in writing.
• The editor knows that marketing is the difference between success and failure, and doesn’t like marketing. Or doesn’t mind it, but can’t or won’t do it well.
• The editor has done so, either under a pen name or perhaps an unpublished work.
• The editor takes more satisfaction in helping and guiding and teaching other people than in creating his or her own projects.
• The editor doesn’t want the public engagement that could come with a reasonably successful book, at least not for the pittance s/he would likely earn from it.
• The editor never got comfortable with the traditional publishing model (writer begs and begs, house condescends to accept the bulk of the revenue).
• The editor isn’t neurotic enough to be a writer. (Okay, I’m sort of kidding. Sort of.)
• Editing and writing require different skill sets and not everyone has both.
• The editor hasn’t got anything original to share.
• The editor is too busy helping others to focus on his/her own book.
• A similar situation exists in many disciplines. Not everyone who can refinish furniture can build it. Not everyone who can repair a car can design and build a car.

Some of those apply to me to varying degrees. I’d bet some apply to most editors.