Why can’t there just be a fixed list of courses that do or do not count?

For several reasons: 

1. As noted above (see the entry on How are applications evaluated?), we are interested in ensuring that your overall course plan has a central theme, a good mix of breadth and depth, an acceptable number of higher-level challenging courses, etc. This means that individual courses can only ever be considered in the context of your overall schedule.
    
2. Also in this vein, some borderline courses may count for some students but not others, depending on their overall course themes, and perhaps also their pre-arranged commitment to write their final paper on a topic related to cognitive science.
    
3. Some courses may only count for the major when they are taught in certain ways, or with certain curricula – since their content can differ considerable from year to year or from professor to professor.