Mostly because doing something "great" typically means winning some sort of competition, and when the talent pool is deep enough, hard work becomes a significant determining factor, at which point the arms race is on, and whoever is most willing to destroy themselves is the winner. When the stakes are high enough, and the child's will falters, the adults around them are ready to impose a will of their own.
It's interesting. On the one hand, our society is content push its children into situations where the maturity needed to cope with the stress levels is typically beyond their years. Yet on the other hand, we infantilize them. For example, parents often don't let 8- or 9-year-old kids walk home from school alone, play outside alone, or go to the park alone out of an unjustified fear of stranger abductions. And then we helicopter them and/or tiger parent them. We also tell them about the complete and utter importance of COLLEGE or IVY LEAGUE SCHOOLS!! at very young ages and expect them to cope with the increasing pressure being piled onto them every year. In the words of a Stanley Teitel, former principal of Stuyvesent High: "Welcome Freshman. For the next 4 years, pick 2 out of the following 3: friends, good grades, sleep." Etc.
All this means that kids may have to cope with adult-level stress before they've even been allowed to walk home from school alone or had a first summer job.
This seems...wrong.
