I've wondered that myself but honestly - it's something I'd leave up to shrinks to figure out
At the same time, a rape fantasy has something that a real live rape doesn't - control. You can stop it at any time, you can decide on what they do to you and what they look like. And many a rape victim actually goes into BDSM to regain some semblance of control through recreating what they went through.
In essence, the one thing that rape fantasies don't share with rape is the actual part where permission isn't granted, and your control to stop anything you don't want happening is taken away. And it is exactly that part which creates the difference in experience - from sexually aroused to completely terrified and traumatised.
Which...makes you wonder indeed if this is a fantasy that is common in order to come to terms with the lack of control some women feel when dealing with men and the insecurity of what will happen, perhaps. A way of preparing the body for something you fear might happen. It certainly has elements of how you would treat a phobia, from what I can tell. But again - that is one for the shrinks.