You get long checkout lines. Even in many places, the express lines are slow. We emphasize speed, speed, speed. When something slows you down, it's irritating. Population leads to slowing us down.
Frustration leads to aggression.
The way things are going right now, I don't think it will get better anytime soon. No one's addressing the problem. I don't agree with you. It's not clear that discipline has declined as much as you imply in your question. For example, almost every teacher training college in America teaches teachers classroom discipline and management.