Popular franchises made into video games... All the Marvel and DC comic games and the new Hollywood movie franchises made into games. Why do you buy them? I generally do not. Ever. Once or twice in the past, but only if the game is a good one, do I buy it... by video game standards, not by a fan's wishful thinking.
The only big franchise games that turned out to be great this generation that I could think of were War For Cybertron and Arkham Asylum. Anything else should be added to that "list?"