Children of over-protective disciplinarians still end up selfish and bratty
Helicopter parents have fooled themselves into believing that their kids will turn out well-mannered and considerate if they could only remove them from "bad influences" in their peer group, and insist on strict rules and correction of bad behavior.
The drive to curb their kids' exposure to bad influences in practice leads them to shelter them from all contact with their peers -- it's not like the parent can tell in advance who'll end up being a good or bad influence. They might choose a house in an area where fewer Bad Influences live (wink wink), but even in their mostly-white middle-class neighborhood, you still never know.
So the end result is the common practice these days of locking kids up indoors all day, with the occasional time outside in the back yard (still isolated from peers, though), and the rare contact with peers being planned out and supervised by grown-ups (the "play date").
Parents today also spend a lot more effort on elaborately correcting their kids. I don't mean only when they do something really wrong -- I remember getting corrected plenty for that -- but having every little act being micro-managed and supervised by the parents to make sure it passes a threshold of politeness and appropriateness. Parents are always telling their kids what to do these days, for every little thing.
Why then do these children grow up to be such self-centered, tantrum-throwing brats? People have been saying that for awhile once Millennials started entering the workplace as late teenagers to get their first job experience. All that effort by their helicopter parents apparently did nothing to give them good manners.
The simple reason is that socialization depends on socializing. Kids know that their parents are never going to throw them out of the house, more or less no matter what they do wrong, because the parents' love, tolerance, and so on, is fairly unconditional. Children don't have to earn their keep withn the family.
Therefore, if the only or primarily source of feedback is the child's own parents, they won't get very strong correction during development, however much frequent nagging and lecturing they may endure. Thus they will persist in their bratty behavior, which can stick well into adulthood. It's one of those "sensitive window" things, although the window is pretty long -- all of childhood and adolescence. But by the time helicopter parents let their kids do their own thing at age 30, it's too late to learn how to act properly around others.
In contrast, children whose parents encourage them early on to socialize with others their age, wind up getting lots of harsh negative feedback. If you act like a brat around a genetic stranger, they'll kick you out of the group, talk shit about you behind your back to your friends, threatening further consequences, and perhaps even result to violence. Parents hold back so much when it comes to punishment -- maybe a thrashing on the butt with a belt is as bad as it tends to get. Pull that same stunt with someone who isn't family, though, and they may try to pound your face in.
This peer correction is a slow and steady process, but ultimately the kid's behavior is shaped to fall within a range acceptable to the peer group he belongs to. Because everyone else in the group doesn't think the kid is someone special, they won't allow really selfish behavior or panicky whining upon receiving punishment. The kid grows to consider others more and becomes more accepting of their punishment when they've screwed up.
Human beings did not evolve in a literate, mediated world, so all this has to take place in face-to-face interactions. Parents are really naive, bordering on stupid, if they think that their kid's peers can socialize him online or through texting. Just look at a 12 year-old's internet comments or their mouthing off in online multiplayer video games.
So, by exaggerating the threat posed by bad influences, even within their well behaved neighborhood, helicopter parents have denied their kids the benefits of interacting with everyone else in the peer group. It can be no surprise that these kids end up socially, emotionally, and morally immature.