Only because nobody has ever had the vision and cajones to take the idea seriously. The only times its really been done are in Saturday morning kids shows, a couple schlocky horror flicks, and a Godzilla movie. If someone (such as Whedon, who already masterfully intertwined several franchises with Avengers) were to really invest themselves into creating a legitimate story that brought these two realities together, the result could be absolutely mind blowing. However, like I said in my earlier post, I wouldn't want this to be one of the actual Star Wars episode, that would just be stupid. But if it was its own spinoff film, or even Avengers 3, I would be very, very interested.