I suspect the reason is that most late-teens/early-twenty-somethings are not responsible/emotionally mature enough to put in the required amount of work in the relatively free environment of university where nobody is checking if you’re doing your homework or show up to class.
Related, for me, was that high school just wasn't very challenging. I got As without ever really studying or feeling that I was working very hard. I took that approach into university and it worked for my freshman and most of my sophomore courses. Then things got actually tough and I realized I could not just intuit my way through exams, and I had never really learned how to study.
Failing to realise that school was just about learning how to learn was also the main mistake I made, for the same reason as you. I was always good at soaking up stuff I found interesting, but to this day learning something because I have to is hard, awful work.
