Those girls and boys, may come from poor families, often first generation or totally new immigrants..Or they may be children of drug/alcohol addicts, "rednecks" , minorities, etc. They have no proper education, many don't even understand that education is much more important than "love".Due to the situation they live in, they fail to see the opportunities available if they get a good education. The sad thing is that they actually have the freedom and opportunity to get this education and become successful, make money and are therefore able to provide for their children, bring them up properly ,so that the circle of early marriage, poverty and teen pregnancy is broken.
Another one of the differences between the kids, in this instance, the US, and other, above mentioned countries.