Pizza, burger and wafers are popularly called as fast food or junk food. Why ?

Junk food refers to the food having low nutritional value and with high calories. It is of not any importance, i.e. why it is referred as junk food. Harmful effects of junk food includes heart diseases, high cholesterol level, lack of energy and others. Pizza, burger and wafers lack nutiritonal value, so are called as fast food or junk food.