Why are Batman Arkham games so short?

So I was wondering why all the Batman Arkham main story lines are so short. All the games are absolutely amazing and the gameplay is terrific and the story lines are awesome... however they're too short... The games are open world games and the main story lines overall take about 12 hours to beat. Compare that to other open world games like Zelda, GTA and Skyrim the story line is very short. I mean if the side quests in Arkham City were intertwined with the main storyline I think it would've been longer but still. So all 3 of the Arkham game story lines were way too short. It's kinda annoying because the games are of suck high quality. The gameplay, story lines, sidequests...