Personally, I think it's abdication of parental responsibilities. Too many parents don't actually want to, or don't have the ability to "parent" properly, so rely on others to do the "hard work", whether it's the nursery staff, teachers, or wider society picking up the pieces.
Yes, screens are part of it, but that comes back to parents abdicating their responsibilities by putting their child in front of a screen rather than doing things with them, talking to them, teaching them things, etc. Just the easy option really.
It's not new, but each generation seems to get worse. If you've had parents yourself who didn't put much effort into nurturing you and abandoned you to nurseries, child minders, relatives, etc., then you're going to think that's normal and do the same to your kids, probably even more so, and so it perpetuates.
When the kids don't do as they're told, break rules, are aggressive, etc., the parents don't have the ability to punish them - either they don't believe their little darling could be naughty, or they don't respect the teachers, police, etc and argue about punishments, etc.