1. What are 'core' or 'hardcore' games? I would argue that for example MW2 and ACII are not casual titles, they have been selling well enough no?
2. As others have said, enthusiast/core/harcore gamers spend a lot more on their hobby.
3. A lot of people working in the games industry are huge game fans ('core'/'hardcore') themselves. To keep them at your company, you have to give them tasks that they like, and they want to work on these 'core' or 'hardcore' titles. I know I wouldn't like to work on Imagine: Babiez...