Because few students graduate in under 4 years.
Sure there is. For starters, many top-tier colleges, primarily privates, cap AP credit (for which the minimum score required is a 5) to one semester or one year, if they offer credit at all. Along the same lines, many private colleges do not accept DE courses as transfer credit. Secondly, at some colleges, for some exams, AP credit it given as general credit which may or may not be applied to major requirements or gen ed requirements. Thirdly, it may be logistically impossible to schedule all requirements within 2 (or 3) years. Finally, as mentioned above, 2 years does not equate to a full college experience.
But, of course, what’s important for one student may not be applicable to another, and what’s applicable/important for a student attending a small private college may be different for a large public flagship.