As one of the comments mentioned, GTA4 could be beaten in under 15 hours (although I've never heard of anyone doing it). By the time I had beaten the storyline for the game, I had clocked in 50 hours. 8-10 hours is a general playtime based on a variety of playing speeds. There's a record in the Guinness Book of World Records for how fast a person beat Halo 2 on Legendary, but that doesn't mean that that's the general length for the campaign. When you consider just how much detail goes into every new environment and battlefield, and the kind of work needed to make them, it's pretty pointless to demonstrate what was obviously a pissing contest to see how fast the campaign could be finished.
Lots of shooters have short campaigns. They're not RPGs, after all. Shooters are so online-oriented that a long campaign mode just isn't important to developers anymore. Three of the most popular shooter series on the market, Halo, Gears of War, and Call of Duty aren't known for their grand and long-winding campaigns either.