they are too busy doing other things?
"Well, there's ballet on Monday, basketball on Tuesday, Wednesday it is piano lesson, Thursdays I do swimming and Friday is Guides and then on Saturday mornings I have to play for the school team and Sundays we have to go and watch my brother play football." Phew! Wild social life. This is the weekly timetable of an eleven year old of my acquaintance. She goes to the local state primary school and her friends do much the same as she does. If they are not doing that then they are in 'after-school care' where, apart from homework, they are not expected or encouraged to sit and read. Doing that would be regarded as anti-social and questions would almost certainly be asked about social adjustment.
All this activity is designed to keep her busy and occupied and under the supervision of an adult at all times. When she is not doing those things she is busy doing homework, texting her friends and getting ready for the next exam or test or badge. It is all highly competitive. I thought she was exceptional - until I discovered that all but two of her classmates were similarly occupied. One of her teachers assured me that it was "pretty much normal" for local children in our "pretty much middle-class" area.
It does not leave much time for "just reading".