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?

Loss of function mutations in any of the glycolytic enzyme would be fatal for  the embryo and the embryo will not survive.

Explanation:

Glycolysis is the first stage of cellular respiration and is a central metabolic pathway for energy production. Glycolysis is also the basic pathway for several other metabolic pathways. Mutation in any of the glycolytic enzymes would not allow the survival of the embryo and the embryo will die in early stages only.

Glycolysis is responsible for the large flux of carbon and is the sole source of energy for several types of cells and tissues such as the brain, sperms, RBCs, etc. Lethal mutations that affect any of the central processes do not allow survival of the organisms.