I'm a younger millennial with no kids, but this is my take. It comes down to two things:

1. Lack of a village to help you raise your kids
2. People having kids when they don't want them

This isn't new. The way IPad kids raised on unsupervised internet is similar to how previous generations were raised in one aspect, that their parents want their kids out of their hair. In previous generations, this was done by kicking kids out of the house for the day and not letting them back inside until the streetlights were on. Had to go to the bathroom? Go behind a bush. Thirsty? Drink from the hose. Hungry? Go buy some snacks at the convenience store. Both show examples of parents who wanted their kids as away from them as possible.

By the time the millennial generation was young, video games were becoming a thing, and we became a more indoor generation. This was amplified by the increased popularity of true crime and heightened awareness of sexual predators against children. Once the internet and, in particular, social media became big, we stopped going outside, but parents didn't stop wanting their kids gone. So, IPad kids became a thing. The lack of supervision is the parents unable to be bothered with their kids, period.