Because they have azzholes for parents.

Kids drop out because education has a low priority in the home and they are usually raised by one parent. And when parent(s) are unsuccessful in school, they tend to pass on their dislike for it to their children. These children in turn become behavior problems because they can't keep up with the work required do to suspension, problems in the home, and too many unexcused absences. So they hang around until they turn sixteen, then quit. Some quit before sixteen because they have given up.