What is the mentioned cause of the obesity problem of children?

(21)Many studies show that children around the world are becoming less active and less physically fit. The result is an increase in obesity, the condition of being severely overweight and other diseases.
    It is a major concern for public health officials. And it may be that fitness levels affect more than physical health. For students, it may also affect test scores.(22)Recently more than 700 children in Redondo Beach, California were walking to school with parents and, in some cases, even their dogs. The students do this once a week. The children may not know it but they walk to help reduce childhood obesity. It is also meant to increase their activity, or fitness level.