I wonder it it's maybe because parents are just stuggling really badly at home for all sorts of reasons and the kids end up on screens a lot. It might be poverty, or illness or whatever, but kids can end up on screens for a long time if parents are just really up against it.
I know because this happened with me when I had a long illness and it did affect my son's development. I didn't mean it to happen, but it can be very hard when the NHS isn't coping and the parents can't access healthcare.
The problem is that once they are behind, the school system drives very hard and it can be so difficult to catch up. The pace is just relentless, and so kids who start at a disadvantage aren't necessarily helped, but can just be driven lower.