Probably because IB courses’ content is less aligned with typical college frosh courses in US colleges and universities.
There may also be options and variations for any given IB subject. For example, even math can have considerable variation, as shown by the advanced placement recommended by one math department for various kinds of IB math scores: https://math.berkeley.edu/courses/choosing/high-school-exam-credits (note: 1A, 1B = calculus for math, physics, engineering, etc.; 10A, 10B = calculus, statistics, etc. for biology and pre-med; 16A, 16B = calculus for business).