How much of it do you remember now? How much of it do you care to remember long term? I know you weren't alone, I was one of those kids too, at times or certain tests. For most people, it was about pride, being able to say you didn't study or social standing and "coolness". Later in life I learned I have ADHD and that went a long way to explain a lot of my bad habits. I think it depends on the level of curriculum too. 11th and 12th, the last two years of HS for me, six out of my eight classes were AP/college weighted, the only expections were english and physical education.

Teachers absolutely tested stuff they didn't teach and they barely assigned homework, there was a lot of self-learning. I think it's relatively uncommon, but in 12th I took AP Physics C, calculus based physics, and we did both semesters over the school year, both Mech and E&M. We matched college pacing, and on both AP test I got 5s. I am so grateful to that teacher for getting that experience before starting college, he had a PhD in theortical physics. My science education was filled with teachers with PhDs while I was in high school.

People absolutely have to study when they are being challenged at an appropriate level. If I was in a non-accelerated curriculum, I think I would have agreed with you, because it would have matched my experience. But I took my first AP class in 10th grade, the second year of high school. There's no way to be successful in a hard science AP class without studying in one way or another. I remember some of my friends used to do the homework from one class in another and then say they didn't study. I am not disagreeing with your experience, I am disagreeing with your generalization.