For the same reason they tell their children that it is bad to lie and punish them when they lie. Because there is an enforced separation between the values people have and the values they espouse , and this separation is especially sharp when it comes to raising children. Wherever there is coercion, lies follow as certainly as night follows day. That is why, in our society, children lie all the time and the parents tell them that lying is bad and punish them for it, even though the parents themselves lie all the time too and know that it is not true that it is always wrong to lie. Not only do parents lie all the time to their children, they often punish their children for not lying.
Most parents force their children to lie.
For example, they insist that their children express gratitude they don't feel, for gifts they don't want. Imagine what would happen if in response to a question about the meal Great Aunt Griselda has served, the children answer truthfully, that it was the most disgusting meal they have ever had the misfortune to be served. Keeping a confidence, for example, often involves lying. Cher is said to be very angry that Mary just couldn't bring herself to lie to protect Sonny's reputation in death. Mary Bono admitted that perhaps she should have refused to answer some of the questions.