- High school classes are typically too easy
- So kids develop poor study habits which don't serve them well for college material
- And most high school teachers are bad at getting kids excited about the subject because they're exhausted themselves from babysitting and treat the work as a job. College professors can be bad at "teaching" but for different reasons (being researchers first and foremost). This disconnect in the reasons for bad education being different in environments is also not taught well to kids ahead of time (because who in this formula would? Requires good parenting or very self-conscious teachers at all levels).
There are definitely exceptions to this rule, but they are too few to solve the overarching problems.