Because they compare video games to the cream of movies, not the B level efforts, and games lose out in that comparison. Is there any game on par with A Clockwork Orange, American Psycho, or The Accused when it comes to integrating sexual violence, themes, and subtext? If so, you can probably count them on less than one hand. Movies like Zombie Diaries 2 are considered "trash" or "exploitation" and not on the mind of critics or commentators attempting to make comparisons.
To sum: Video game writing hasn't reached the craft or sophistication of the top scripts or novels, which are what they get compared to. How games handle rape, crime, and sexuality is crude, exploitive, and unintelligent in comparison. That's why they are treated differently imo.