Let's face it. "Video game movie" isn't exactly a phrase that brings to mind cinematic masterpiece. In fact, I'll bet you're imagining stuff like Street Fighter: The Legend of Chun Li or Alone In The Dark, and you're probably at least a little bit wary about Prince of Persia, even if the trailers look good.
So why does this happen so often? The biggest, easiest, and stupidest sounding reason is because video games and movies are two different things.