Nothing is 100%, but some are more stupid than others.
WHY? Variety of reasons. Fear of litigation means fear of going outside one's door, lack of parental motivation / discipline, leading to lack of motivation / interest/stimulation.
My wife somehow seemed to finish up teaching in schools with a less intelligent pupil intake. She said that the day often consisted of animal training with a little light teaching thrown, if you were lucky.
Think that's exaggeration? Some of her colleagues were actually assaulted! A complaint was made that she made "My girl work in the afternoon. She doesn't come to school to work all day". The question that the mother would want her child to become qualified and get a good job was met with a blank stare.
On the other hand, I know of a 15 yo who got extra work, after work experience, because he was better than an existing employee.
A 17 yo from a pretty normal home has received offers from three universities.
An undergraduate who will be able to have a career on the comedy circuit, if all else fails. (Not the first by a long way)
The fear is that the chasm between the "haves" and "have nots" will widen because of their outlook on life.
Swings and roundabouts, horse for courses, and all that.
Howard