i think it’s that our brains like to do mostly easy things and learning new words is not easy, it has to fire up those neural pathways for that and create new connections for not a lot gratification for many people, simmiliar to how children often need external motivation to start to learn an instrument or other complex endeavors, because the fun part of it requires a suprising amount of practice and expertise, and if no one is pushing them to get through a specific book they’ll pick an “easier” read or something else entirely for entertainment.