Because children are too wet behind the ears to realize yet how important an education is, and that they are lucky to be in a country in which one is available to them. Adults have been around longer, so would likely realize the opportunity their children have for an education, and realize why it is important for a successful lifestyle.
everyone needs motivation. it is a lot more fun to play games, sleep in, and do nothing than to go to school, get up early, and study and homework. if you didn't have to work to pay bills etc, would you? have you ever played hooky from work- called in sick when you weren't just because you were unmotivated to go to work? children are much less self-disciplined than adults, so we have to motivate them.
Because kids don't quite see the value of the education yet. They see annoying teachers and torn books and cramped stuffy classrooms and staple cuts and paper cuts and clocks that move slowly. They don't see a nice future or a nice career. Not yet.