It's about cultural preference not ability. School does a positively terrible job of making it easy and comfortable for kids to learn their stuff, and the more recent "student-centered", "progressive" or "constructivist" approaches to learning actually make this a lot worse not better since they heavily imply (as a matter of practical implementation, if perhaps not always in theory) that the teacher shouldn't even act to provide helpful guidance and direction. Many school pupils do the short-term reasonable thing (as they see it) and just do not bother unless they're motivated by some sort of independent interest, or pushed by their household environment to be more successful (the "tiger parenting" approach).