They also read for their own enjoyment, which provides a powerful example. Sometimes one stays home not because the parents are rich, but because they have made that choice. They have given up a lot in order to provide their children the beginnings of an education at what the researchers say is the most crucial period of a child's learning life. The point here is to emphasize that these are normal kids. I think when you read the research you will agree that normal kids can acquire signifanct knowledge very early.
Most people know that it's easier to acquire language as a child than as an adult.
The research indicates that other basic information can be just as easily learned, providing care is given to provide the atmosphere for learning. There is definitely a relationship between the quality of care a baby gets and later achievment. But providing for that crucial period of education isn't always possible. Too often, both parents have to work just to provide food, shelter and clothing. There is less time to give to children.