Why aren't harrison's parents more emotionally impacted by the death of their son?

In Vonnegut's Harrison Bergeron, Harrison's parents are incapable of being emotionally impacted by their son's death due to their numerous handicaps.

Because they are so handicapped, they are unable to concentrate or focus on something for more than a moment. Their handicaps prevent them from thinking clearly or remembering details. As a result, although they initially feel sad, they quickly forget what happened and continue watching television.