There are a variety of fairly common human genetic diseases in which enzymes required for the breakdown of fructose, lactose, or sucrose are defective. however, there are very few cases of people having a genetic disease in which one of the enzymes of glycolysis is severely affected. why do you suppose such mutations are seen so rarely?

This is because such as person would rarely survive. Such a mutation on such a critical enzyme would literarily stifle life out of the person. In fact, if such a mutation occurs in the early stages in the growth and development of an embryo or fetus, the child would barely develop but would die due to lack of energy to power all biological process of life. GLusoce is the main source of energy for life in most organisms.