Simply because the medical researchers of the world, until very recently, were white northern Europeans. And whenever they saw PLI, they saw it in adults. It wasn't until the 1970s that most doctors began to become aware that in many, if not most, Native American, African, and Asian populations, lactase begins to decline at about the age of weaning. The fact that many doctors even today seem to have no idea that this is true would be just an embarrassment for the medical community if it didn't have real effects in the real world of the United States. Since immigration to the U.S. is increasing precisely from those areas of the world in which PLI starts in children, the Lactose Intolerance of many children is not being recognized.
Lactase is made in a delicate and vulnerable part of the small intestine.
If the intestine is damaged, so is the body's ability to manufacture lactase. This condition is called Secondary Lactose Intolerance to distinguish it from the natural gradual loss of lactase ability. The list of conditions that can bring on SLI is a long one. Operations to the small intestine, especially those that remove portions of it, are also a leading cause. But some drugs can produce it, and so can long-term trauma to the body as in alcoholism.