The reason children aren't behaving (at least in the U.S.) is because a large number of parents never practice parenting and never seek out advice from reliable resources.

For example-- I've heard numerous vacuous rationalizations from parents young and old for spanking their children. What's the research say on spanking? The vast majority over decades says that it doesn't improve behavior and in fact increases the likelihood of aggression in children.

So why would parents continue doing it? It's the same reason an ineffectual guitarist keeps starting over on Stairway to Heaven. Because, "Gosh, I had it perfect once out of 200 failures in my bedroom by myself last week."

So it is with parents: "Gosh, I don't get why they're acting out, they were so good that one time out of 200 outbursts while I was paying a moderate amount of attention to them last week."

Edit: clarification